Semiconductor device and semiconductor memory device

ABSTRACT

An object is at least one of a longer data retention period of a memory circuit, a reduction in power consumption, a smaller circuit area, and an increase in the number of times written data can be read to one data writing operation. The memory circuit has a first field-effect transistor, a second field-effect transistor, and a rectifier element including a pair of current terminals. A data signal is input to one of a source and a drain of the first field-effect transistor. A gate of the second field-effect transistor is electrically connected to the other of the source and the drain of the first field-effect transistor. One of the pair of current terminals of the rectifier element is electrically connected to a source or a drain of the second field-effect transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice. Further, one embodiment of the present invention relates to asemiconductor memory device.

2. Description of the Related Art

In recent years, semiconductor devices each including a memory circuitin which data can be written and erased and the data can be retained fora certain period have been developed.

As examples of the above semiconductor device, a dynamic random accessmemory (also referred to as a DRAM) (for example, see Patent Document 1)and a static random access memory (also referred to as an SRAM) (forexample, see Patent Document 2) can be given.

A DRAM disclosed in Patent Document 1 is provided with a memory cellincluding one transistor and one capacitor. The DRAM disclosed in PatentDocument 1 retains data by turning the transistor on and accumulatingelectric charge in the capacitor.

An SRAM disclosed in Patent Document 2 is provided with a memory cellincluding six transistors.

REFERENCE

[Patent Document]

[Patent Document 1] Japanese Translation of PCT InternationalApplication No. 2004-529502

[Patent Document 2] PCT International Publication No. 2008/114716

SUMMARY OF THE INVENTION

However, a conventional DRAM such as that disclosed in Patent Document 1has a problem of high power consumption due to a short data retentionperiod; for example, a rewrite operation of the same data (also referredto as a refresh operation) needs to be performed several tens of timesper second. In addition, a conventional DRAM such as that disclosed inPatent Document 1 also has a problem in that the number of times datacan be read to one data writing operation is one. This is because oncethe data is read, the data is lost.

Further, a conventional SRAM such as that disclosed in Patent Document 2also has a problem of high power consumption, as in the case of aconventional DRAM, because power supply needs to be continued duringdata retention. In addition, in the case of a conventional SRAM, sincethe number of transistors in a memory cell is large, the area of acircuit is large and integration of the circuits is thus difficult.

An object of one embodiment of the present invention is at least one ofa longer data retention period of a memory circuit, a reduction in powerconsumption, a smaller circuit area, and an increase in the number oftimes written data can be read to one data writing operation.

In one embodiment of the present invention, a memory cell including twotransistors and one rectifier element is provided. With this structure,a reduction in power consumption or an increase in the number of timeswritten data can be read to one data writing operation is achieved whilean increase in a circuit area is suppressed.

Further, in one embodiment of the present invention, at least one of theabove transistors can be a transistor with low off-state current.Moreover, the transistor may include an oxide semiconductor layer whichincludes a region to which a dopant is added. By providing the region towhich the dopant is added for the oxide semiconductor layer of thetransistor, miniaturization of the transistor is achieved.

In one embodiment of the present invention, the oxide semiconductorlayer of the transistor can be formed using a material (also referred toas a crystal with c-axis alignment or c-axis aligned crystal; CAAC)which is non-single-crystal and includes a phase which has a triangular,hexagonal, regular triangular, or regular hexagonal atomic arrangementwhen seen from the direction perpendicular to the a-b plane (alsoreferred to as a plane of a layer) and in which metal atoms are arrangedin a layered manner when seen from the direction perpendicular to thec-axis direction (also referred to as a thickness direction of a layer)or metal atoms and oxygen atoms are arranged in a layered manner whenseen from the direction perpendicular to the c-axis direction. In thiscase, deterioration of the transistor due to light is suppressed.

According to one embodiment of the present invention, a data retentionperiod of a memory circuit can be lengthened. Accordingly, powerconsumption can be reduced. According to one embodiment of the presentinvention, the number of times written data can be read to one datawriting operation can be increased. According to one embodiment of thepresent invention, a circuit area can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates an example of a memory circuit in a semiconductordevice;

FIGS. 2A to 2C illustrate an example of a memory cell array in asemiconductor memory device;

FIGS. 3A to 3C illustrate an example of a memory cell array in asemiconductor memory device;

FIGS. 4A to 4D are cross-sectional schematic views each illustrating anexample of a structure of a transistor;

FIGS. 5A to 5E are cross-sectional schematic views illustrating anexample of a method for manufacturing a transistor;

FIG. 6 is a cross-sectional schematic view illustrating an example of astructure of a memory cell;

FIG. 7 is a block diagram illustrating an example of a structure of asemiconductor memory device;

FIGS. 8A to 8D are schematic views each illustrating an example of anelectronic device;

FIGS. 9A and 9B are circuit diagrams each illustrating an example of astructure of a conventional memory;

FIG. 10 is a block diagram illustrating an example of a structure of amobile phone;

FIG. 11 is a block diagram illustrating an example of a structure of amemory;

FIG. 12 is a block diagram illustrating an example of a structure of ane-book reader;

FIGS. 13A to 13E each illustrate a structure of an oxide materialaccording to one embodiment of the present invention;

FIGS. 14A to 14C illustrate a structure of an oxide material accordingto one embodiment of the present invention;

FIGS. 15A to 15C illustrate a structure of an oxide material accordingto one embodiment of the present invention;

FIG. 16 shows gate voltage dependence of mobility obtained bycalculation;

FIGS. 17A to 17C each show gate voltage dependence of drain current andmobility of a transistor, which is obtained by calculation;

FIGS. 18A to 18C each show gate voltage dependence of drain current andmobility of a transistor, which is obtained by calculation;

FIGS. 19A to 19C each show gate voltage dependence of drain current andmobility of a transistor, which is obtained by calculation;

FIGS. 20A and 20B each illustrate a cross-sectional structure of atransistor used for calculation;

FIGS. 21A to 21C are graphs each showing characteristics of a transistorincluding an oxide semiconductor film;

FIGS. 22A and 22B are graphs each showing V_(g)-I_(d) characteristics ofa transistor of Sample 1 which is subjected to a BT test;

FIGS. 23A and 23B are graphs each showing V_(g)-I_(d) characteristics ofa transistor of Sample 2 which is subjected to a BT test;

FIG. 24 shows XRD spectra of Sample A and Sample B;

FIG. 25 is a graph showing a relation between off-state current of atransistor and substrate temperature in measurement;

FIG. 26 is a graph showing V_(g) dependence of I_(d) and field-effectmobility;

FIG. 27A is a graph showing a relation between substrate temperature andthreshold voltage and FIG. 27B is a graph showing a relation betweensubstrate temperature and field-effect mobility;

FIGS. 28A and 28B are a top view and a cross-sectional view of atransistor; and

FIGS. 29A and 29B are a top view and a cross-sectional view of atransistor.

DETAILED DESCRIPTION OF THE INVENTION

Examples of embodiments of the present invention will be described belowwith reference to the drawings. Note that it will be readily appreciatedby those skilled in the art that details of the embodiments can bemodified in various ways without departing from the spirit and scope ofthe present invention. The present invention is therefore not limited tothe following description of the embodiments.

Note that the details of the embodiments can be combined with each otheras appropriate. In addition, the details of the embodiments can bereplaced with each other.

Ordinal numbers such as “first” and “second” are used in order to avoidconfusion among components, and the number of components is not limitedby the number of ordinal numbers.

Embodiment 1

In this embodiment, an example of a semiconductor device that includes amemory circuit capable of storing data for a certain period isdescribed.

Note that a memory circuit is a circuit capable of retaining electriccharge as data for a certain period.

The example of the semiconductor device in this embodiment includes amemory circuit.

An example of the memory circuit is described with reference to FIG. 1.

First, an example of a structure of the memory circuit in thesemiconductor device in this embodiment is described with reference toFIG. 1.

The memory circuit illustrated in FIG. 1 includes a transistor 111, atransistor 112, and a rectifier element (rct) 113.

Note that the transistor includes two terminals and a current controlterminal for controlling current flowing between the two terminals byvoltage applied. Note that without limitation to the transistor, in anelement, terminals where current flowing therebetween is controlled arealso referred to as current terminals. Two current terminals are alsoreferred to as a pair of current terminals and such two currentterminals are also referred to as a first current terminal and a secondcurrent terminal.

Further, a field-effect transistor can be used as the transistor, forexample. In a field-effect transistor, a first current terminal, asecond current terminal, and a current control terminal are one of asource and a drain, the other of the source and the drain, and a gate,respectively.

Depending on a structure or operation conditions of a transistor, asource and a drain of the transistor are interchanged in some cases.

Further, the rectifier element includes a pair of current terminals andis brought into conduction when the pair of current terminals arebrought into conduction in accordance with voltage applied between thepair of current terminals.

The term “voltage” generally means a difference between potentials attwo points (also referred to as a potential difference). However, levelsof voltage and potentials are represented by volts (V) in a circuitdiagram or the like in some cases, so that it is difficult todistinguish them. Thus, in this specification, a potential differencebetween a potential at one point and a potential to be a reference (alsoreferred to as a reference potential) is used as voltage at the point insome cases unless otherwise specified.

A data signal is input to one of a source and a drain of the transistor111. The transistor 111 serves as a selection transistor for determiningwhether or not data is written to the memory circuit. A writingselection signal may be input to a gate of the transistor 111. A writingselection signal is a pulse signal for determining whether or not datais written to the memory circuit.

As the transistor 111, a transistor including an oxide semiconductorlayer in which a channel is formed can be used, for example.

The band gap of the oxide semiconductor layer is larger than that ofsilicon and for example, 2 eV or more, preferably 2.5 eV or more, farpreferably 3 eV or more.

Further, such a transistor which includes the oxide semiconductor layercan have lower off-state current than a conventional field-effecttransistor which uses silicon.

It is possible to use, as the transistor 111, a transistor whichincludes an oxide semiconductor layer having a pair of regions which areseparated from each other and to which a dopant is added. In thetransistor which includes the oxide semiconductor layer having the pairof regions to which the dopant is added, a channel is formed between thepair of regions of the oxide semiconductor layer, to which the dopant isadded. It is preferable that resistance of the pair of regions to whichthe dopant is added be lower than that of a region in which the channelis formed (also referred to as a channel formation region). With the useof the transistor which includes the oxide semiconductor layer havingthe pair of regions to which the dopant is added, resistance between theregion in which the channel is formed (also referred to as the channelformation region) and a source or a drain of the transistor can be low,so that a reduction in the area of the transistor (also referred to asminiaturization of the transistor) is possible.

As the oxide semiconductor layer, it is possible to use, for example, anoxide semiconductor layer formed using a material which isnon-single-crystal and includes a phase which has a triangular,hexagonal, regular triangular, or regular hexagonal atomic arrangementwhen seen from the direction perpendicular to the a-b plane and in whichmetal atoms are arranged in a layered manner when seen from thedirection perpendicular to the c-axis direction or metal atoms andoxygen atoms are arranged in a layered manner when seen from thedirection perpendicular to the c-axis direction. By using an oxidesemiconductor layer including CAAC as a layer in which a channel of atransistor is formed (also referred to as a channel formation layer),deterioration of the transistor due to light, for example, can besuppressed.

A gate of the transistor 112 is electrically connected to the other ofthe source and the drain of the transistor 111.

Note that as long as there exists a period during which two or morecomponents are electrically connected, the two or more components can besaid to be electrically connected.

As the transistor 112, for example, it is possible to use a transistorwhich includes a semiconductor layer including a semiconductor belongingto Group 14 of the periodic table (e.g., silicon) or the above oxidesemiconductor layer as a layer in which a channel is formed.

Note that the semiconductor layer including a semiconductor belonging toGroup 14 may be a single crystal semiconductor layer, a polycrystallinesemiconductor layer, a microcrystalline semiconductor layer, or anamorphous semiconductor layer.

One of the pair of current terminals of the rectifier element 113 iselectrically connected to a source or a drain of the transistor 112.Here, the source or the drain of the transistor 112, to which one of thepair of current terminals of the rectifier element 113 is notelectrically connected, is “one of the source and the drain of thetransistor 112”; the source or the drain of the transistor 112, to whichone of the pair of current terminals of the rectifier element 113 iselectrically connected, is “the other of the source and the drain of thetransistor 112”. A reading selection signal may be input to the other ofthe pair of current terminals of the rectifier element 113. A readingselection signal is a pulse signal for determining whether or not datais read from the memory circuit. The rectifier element 113 is aswitching element which determines whether or not data is read from thememory circuit.

Next, an example of a method for driving the memory circuit in FIG. 1 isdescribed.

In the case where data is written to the memory circuit, the transistor111 is turned on. For example, the transistor 111 is turned on bychanging the voltage of the gate of the transistor 111. Further, at thistime, a value of the voltage of the other of the pair of currentterminals of the rectifier element 113 and a value of the voltage of oneof the source and the drain of the transistor 112 are set such that therectifier element 113 is brought out of conduction. For example, it ispreferable that the value of the voltage of the other of the pair ofcurrent terminals of the rectifier element 113 be equivalent to thevalue of the voltage of one of the source and the drain of thetransistor 112.

When the transistor 111 is on, a data signal is input through the sourceand the drain of the transistor 111, and the voltage of the gate of thetransistor 112 has a value based on the voltage of the data signalinput. Thus, data is written to the memory circuit.

After that, when the transistor 111 is turned off, the gate of thetransistor 112 is in a floating state, so that the voltage of the gateof the transistor 112 is held for a predetermined period.

In the case where data is read from the memory circuit, the value of thevoltage of the other of the pair of current terminals of the rectifierelement 113 is set such that an absolute value of a difference betweenthe voltage of one of the source and the drain of the transistor 112 andthe voltage of the other of the pair of current terminals of therectifier element 113 is larger than or equal to a value of voltagenecessary for bringing the rectifier element 113 into conduction. Thevoltage at this time is also referred to as a reading voltage.

Resistance between the source and the drain of the transistor 112depends on the voltage of the gate of the transistor 112. In accordancewith current flowing between the source and the drain of the transistor112, whether or not the rectifier element 113 is brought into conductionis determined and the voltage of the other of the pair of currentterminals of the rectifier element 113 is set. For example, the voltageof the other of the pair of current terminals of the rectifier element113 changes when the transistor 112 is turned on and the rectifierelement 113 is brought into conduction. Therefore, in the memorycircuit, the voltage of the other of the pair of current terminals ofthe rectifier element 113 can be read from the memory circuit as data.Further, during a period in which the transistor 111 is in an off state,the voltage of the gate of the transistor 112 is held for apredetermined period; thus, the voltage of the other of the pair ofcurrent terminals of the rectifier element 113 can be read from thememory circuit as data more than once. The above is description of anexample of a method for driving the semiconductor device illustrated inFIG. 1.

The above is description of an example of the semiconductor device inthis embodiment.

In an example of the semiconductor device in this embodiment, a memorycircuit can include two field-effect transistors and one rectifierelement, whereby the area of the circuit can be made smaller than thatof a memory cell of a conventional SRAM, for example.

In an example of the semiconductor device in this embodiment, the numberof times written data can be read to one data writing operation can beincreased by setting the voltage of a gate of a second field-effecttransistor, which is electrically connected to a source or a drain of afirst field-effect transistor to a value based on the voltage of a datasignal.

In addition, in an example of the semiconductor device in thisembodiment, a field-effect transistor with low off-state current is usedas the first field-effect transistor, whereby a data retention periodcan be lengthened. Therefore, even if a refresh operation is needed, forexample, the number of times of refresh operations can be smaller; thus,power consumption can be reduced and the capacitance of a capacitoradded to the gate of the second field-effect transistor can be reducedor the capacitor can be omitted.

Embodiment 2

In this embodiment, an example of a semiconductor memory device isdescribed as an example of the semiconductor device in the aboveembodiment.

An example of a semiconductor memory device in this embodiment includesa memory cell array including a plurality of memory cells arranged in amatrix of i rows (i is a natural number of 2 or more) and j columns (jis a natural number). The memory cell corresponds to the memory circuitin the semiconductor device in the above embodiment.

An example of the memory cell array in the semiconductor memory devicein this embodiment is described with reference to FIGS. 2A to 2C.

First, an example of the circuit structure of the memory cell array inthe semiconductor memory device in this embodiment is described withreference to FIG. 2A.

A memory cell array illustrated in FIG. 2A includes a plurality ofmemory cells 200 arranged in a matrix of i rows and j columns; j wirings201 (wirings 201_1 to 201 _(—) j); i wirings 202 (wirings 202_1 to 202_(—) i); i wirings 203 (wirings 203_1 to 203 _(—) i); and j wirings 204(wirings 204_1 to 204 _(—) j). Note that in description of the memorycell array illustrated in FIG. 2A, i is a natural number of 3 or moreand j is a natural number of 3 or more, for convenience.

The memory cell 200 stores data.

The voltage of the wirings 201_1 to 201 _(—) j is controlled by, forexample, a driver circuit including a decoder. The wirings 201_1 to 201_(—) j can be referred to as bit lines or data lines, for example.

The voltage of the wirings 202_1 to 202 _(—) i is controlled by, forexample, a driver circuit including a decoder. The wirings 202_1 to 202_(—) i can be referred to as word lines, for example.

The voltage of the wirings 203_1 to 203 _(—) i is controlled by, forexample, a driver circuit including a decoder. The wirings 203_1 to 203_(—) i can be referred to as source lines, for example.

The voltage of the wirings 204_1 to 204 _(—) j is controlled by, forexample, a driver circuit including a decoder. The wirings 204_1 to 204_(—) j can be referred to as selection lines, for example.

The memory cell 200 (M, N) in an M-th row (M is a natural number of i orless) and an N-th column (N is a natural number of j or less) includes atransistor 211 (M, N), a transistor 212 (M, N), and a rectifier element(also referred to as rct) 213 (M, N).

One of a source and a drain of the transistor 211 (M, N) is electricallyconnected to the wiring 201_N. A gate of the transistor 211 (M, N) iselectrically connected to the wiring 202_M.

The transistor 211 (M, N) determines whether or not data is written.Note that the transistor 211 (M, N) can be referred to as a selectiontransistor, for example.

As the transistor 211 (M, N), a transistor that can be used as thetransistor 111 in the semiconductor device in Embodiment 1 can be used.

One of a source and a drain of the transistor 212 (M, N) is electricallyconnected to the wiring 203_M. A gate of the transistor 212 (M, N) iselectrically connected to the other of the source and the drain of thetransistor 211 (M, N).

The transistor 212 (M, N) sets a value of data to be output. Note thatthe transistor 212 (M, N) can be referred to as an output transistor,for example.

As the transistor 212 (M, N), a transistor that can be used as thetransistor 112 in the semiconductor device in Embodiment 1 can be used.

One of a pair of current terminals of the rectifier element 213 (M, N)is electrically connected to the wiring 204_N. The other of the pair ofcurrent terminals of the rectifier element 213 (M, N) is electricallyconnected to the other of the source and the drain of the transistor 212(M, N).

As the rectifier element 213 (M, N), for example, a diode, or afield-effect transistor whose drain and gate are electrically connectedto each other can be used.

For example, in the case where a diode is used as the rectifier element213 (M, N) and the transistor 212 (M, N) is an n-channel transistor, ananode of the diode serves as one of the pair of current terminals of therectifier element 213 (M, N), and a cathode of the diode serves as theother of the pair of current terminals of the rectifier element 213 (M,N). On the other hand, in the case where the transistor 212 (M, N) is ap-channel transistor, the cathode of the diode serves as one of the pairof current terminals of the rectifier element 213 (M, N), and the anodeof the diode serves as the other of the pair of current terminals of therectifier element 213 (M, N).

Alternatively, when a field-effect transistor whose drain and gate areelectrically connected to each other is used as the rectifier element213 (M, N), for example, one of a source and the drain of thefield-effect transistor serves as one of the pair of current terminalsof the rectifier element 213 (M, N), and the other of the source and thedrain of the field-effect transistor serves as the other of the pair ofcurrent terminals of the rectifier element 213 (M, N).

Further, an example of a method for driving the memory cell array inFIG. 2A is described with reference to FIGS. 2B and 2C. FIGS. 2B and 2Care timing charts each illustrating an example of a method for drivingthe memory cell array in FIG. 2A. Here, the case where data issequentially written to the memory cells 200 (the memory cells 200 (M,1) to 200 (M, j)) in the M-th row and then the data written is read isdescribed as an example; however, the present invention is not limitedto this example and data writing or data reading can be performed forthe memory cells 200 individually.

First, in the case where data is written to the memory cells 200 in theM-th row, the transistors 211 (M, 1) to 211 (M, j) in the memory cells200 in the M-th row are turned on. Note that at this time, the rectifierelements 213 (1, 1) to 213 (i, j) in all the memory cells 200 arepreferably brought out of conduction. By bringing the rectifier elements213 (1, 1) to 213 (i, j) out of conduction, current does not flowthrough the source and the drain of the transistor 212 (M, N) and thepair of current terminals of the rectifier element 213 (M, N) in thememory cell 200 in the M-th row and the N-th column at the time of datawriting, whereby power consumption can be reduced.

For example, in the case where the transistors 211 (M, 1) to 211 (M, j)in the memory cells 200 in the M-th row are n-channel transistors, bysetting the voltage of the wiring 202_M in the M-th row to voltage VH asillustrated in FIG. 2B, the transistors 211 (M, 1) to 211 (M, j) in thememory cells 200 in the M-th row can be turned on. The voltage VH is,for example, voltage having a larger value than reference potential.Further, at this time, the voltage of the wirings 202_1 to 202 _(—) iother than the wiring 202_M in the M-th row (also referred to as thewirings 202_other) is set to voltage VL. Furthermore, at this time, therectifier element 213 (M, N) can be brought out of conduction by settingthe voltage of the wirings 203_1 to 203 _(—) i to the voltage VL andsetting the voltage of the wirings 204_1 to 204 _(—) j to the voltageVL. The voltage VL is, for example, lower than or equal to the referencepotential. At this time, an absolute value of a difference between thevoltage VH and the voltage VL is preferably larger than an absolutevalue of the threshold voltage of the transistor 211 (M, N).

When the transistors 211 (M, 1) to 211 (M, j) in the memory cells 200 inthe M-th row are in an on state, a data signal is input to the memorycells 200 in the M-th row from the wirings 201_1 to 201 _(—) j, so thatthe voltage of the gates of the transistors 212 (M, 1) to 212 (M, j) hasa value based on the voltage of the data signal input, and the memorycells 200 in the M-th row are in a written state.

Then, the transistors 211 (M, 1) to 211 (M, j) in the memory cells 200in the M-th row are turned off, whereby the voltage of each of the gatesof the transistors 212 (M, 1) to 212 (M, j) in the memory cells 200 inthe M-th row is held for a predetermined period.

In addition, when the above-described operation is repeatedly performedfor the memory cells 200 in each row, data can be written to all thememory cells 200.

Further, in the case where data is read from the memory cells 200 in theM-th row, a value of the voltage of the wiring 203_M is set such that anabsolute value of a difference between the voltage of the wiring 203_Min the M-th row and the voltage of each of the wirings 204_1 to 204 _(—)j is larger than or equal to a value of voltage necessary for bringingthe rectifier elements 213 (M, 1) to 213 (M, j) in the memory cells 200in the M-th row into conduction. Further, the voltage of the wirings203_1 to 203 _(—) i other than the wiring 203_M in the M-th row is setequivalent to the voltage of each of the wirings 204_1 to 204 _(—) j,whereby the data can be read only from the memory cells 200 in the M-throw. Further, at this time, the transistors 211 (1, 1) to 211 (i, j) inall the memory cells 200 are turned off.

For example, in the case where the transistors 211 (M, 1) to 211 (M, j)and the transistors 212 (M, 1) to 212 (M, j) in the memory cells 200 inthe M-th row are n-channel transistors, as illustrated in FIG. 2C, thevoltage of the wiring 203_M is set to the voltage VL; the voltage of thewirings 203_1 to 203 _(—) i other than the wiring 203_M (also referredto as the wirings 203_other) is set to the voltage VH; and the voltageof the wirings 204_1 to 204 _(—) j is set to the voltage VH. Further, atthis time, the voltage of the wirings 202_1 to 202 _(—) i is set to thevoltage VL.

Resistance between the source and the drain of each of the transistors212 (M, 1) to 212 (M, j) in the memory cells 200 in the M-th row dependson the voltage of the gate of each of the transistors 212 (M, 1) to 212(M, j). Further, in accordance with current flowing between the sourceand the drain of each of the transistors 212 (M, 1) to 212 (M, j),whether or not the rectifier elements 213 (M, 1) to 213 (M, j) in thememory cells 200 in the M-th row are brought into conduction isdetermined and the voltage of the wirings 204_1 to 204 _(—) j is set.Therefore, when the voltage of the wirings 204_1 to 204 _(—) j at thistime is used as data, the data can be read from the memory cells 200 inthe M-th row. Moreover, during a period in which the transistors 211 (M,1) to 211 (M, j) are in an off state, the voltage of the gates of thetransistors 212 (M, 1) to 212 (M, j) is held for a predetermined period;thus, it is possible to read the voltage of the wirings 204_1 to 204_(—) j as the data from the memory cells 200 in the M-th row more thanonce.

In addition, when the above-described operation is performed for thememory cells 200 in each row, the data can be read from all the memorycells (the memory cells 200 (1, 1) to 200 (i, j)). The above isdescription of an example of a method for driving the semiconductormemory device illustrated in FIG. 2A.

Another example of the semiconductor memory device in this embodiment isdescribed.

Another example of a semiconductor memory device in this embodimentincludes a memory cell array including a plurality of memory cellsarranged in a matrix of i rows and j columns.

Further, an example of the memory cell array in the semiconductor memorydevice in this embodiment is described with reference to FIGS. 3A to 3C.Note that description of the semiconductor memory device described withreference to FIGS. 2A to 2C is referred to as appropriate for portionsthat are the same as those of the semiconductor memory device describedwith reference to FIGS. 2A to 2C.

First, an example of the circuit structure of the memory cell array inthe semiconductor memory device in this embodiment is described withreference to FIG. 3A.

A memory cell array illustrated in FIG. 3A includes the plurality ofmemory cells 200 arranged in a matrix of i rows and j columns; j wirings205 (wirings 205_1 to 205 _(—) j); the i wirings 202 (wirings 202_1 to202 _(—) i); and the i wirings 203 (wirings 203_1 to 203 _(—) i). Notethat in description of the memory cell array illustrated in FIG. 3A, iis a natural number of 3 or more and j is a natural number of 3 or more,for convenience.

The voltage of the wirings 205_1 to 205 _(—) j is controlled by, forexample, a driver circuit including a decoder. The wirings 205_1 to 205_(—) j can be referred to as bit lines or data lines.

The memory cell 200 (M, N) in the M-th row and the N-th column includesthe transistor 211 (M, N), the transistor 212 (M, N), and the rectifierelement 213 (M, N).

One of the source and the drain of the transistor 211 (M, N) iselectrically connected to the wiring 205_N. The gate of the transistor211 (M, N) is electrically connected to the wiring 202_M.

The transistor 211 (M, N) determines whether or not data is input.

One of the source and the drain of the transistor 212 (M, N) iselectrically connected to the wiring 203_M. The gate of the transistor212 (M, N) is electrically connected to the other of the source and thedrain of the transistor 211 (M, N).

The transistor 212 (M, N) sets a value of data to be output.

One of the pair of current terminals of the rectifier element 213 (M, N)is electrically connected to the wiring 205_N. The other of the pair ofcurrent terminals of the rectifier element 213 (M, N) is electricallyconnected to the other of the source and the drain of the transistor 212(M, N).

Further, an example of a method for driving the memory cell array inFIG. 3A is described with reference to FIGS. 3B and 3C. FIGS. 3B and 3Care timing charts each illustrating an example of a method for drivingthe memory cell array in FIG. 3A. Here, the case where data issequentially written to the memory cells 200 (the memory cells 200 (M,1) to 200 (M, j)) in the M-th row and then the data written is read isdescribed as an example; however, the present invention is not limitedto this example and data writing or data reading can be performed forthe memory cells 200 individually.

First, in the case where data is written to the memory cells 200 in theM-th row, the transistors 211 (M, 1) to 211 (M, j) in the memory cells200 in the M-th row are turned on. Note that at this time, the rectifierelements 213 (1, 1) to 213 (i, j) in all the memory cells 200 arepreferably brought out of conduction. By bringing the rectifier elements213 (1, 1) to 213 (i, j) out of conduction, current does not flowthrough the source and the drain of the transistor 212 (M, N) and thepair of current terminals of the rectifier element 213 (M, N) in thememory cell 200 in the M-th row and the N-th column at the time of datawriting, whereby power consumption can be reduced.

For example, in the case where the transistors 211 (M, 1) to 211 (M, j)in the memory cells 200 in the M-th row are n-channel transistors, bysetting the voltage of the wiring 202_M in the M-th row to the voltageVH as illustrated in FIG. 3B, the transistors 211 (M, 1) to 211 (M, j)in the memory cells 200 in the M-th row can be turned on. Further, atthis time, the voltage of the wirings 202_1 to 202 _(—) i other than thewiring 202_M is set to the voltage VL.

When the transistors 211 (M, 1) to 211 (M, j) in the memory cells 200 inthe M-th row are in an on state, a data signal is input to the memorycells 200 in the M-th row from the wirings 205_1 to 205 _(—) j, so thatthe voltage of the gates of the transistors 212 (M, 1) to 212 (M, j) hasa value based on the voltage of the data signal input, and the memorycells 200 in the M-th row are in a written state.

Then, the transistors 211 (M, 1) to 211 (M, j) in the memory cells 200in the M-th row are turned off, whereby the voltage of each of the gatesof the transistors 212 (M, 1) to 212 (M, j) in the memory cells 200 inthe M-th row is held for a predetermined period.

In addition, when the above-described operation is repeatedly performedfor the memory cells 200 in each row, data can be written to all thememory cells 200.

Further, in the case where data is read from the memory cells 200 in theM-th row, a value of the voltage of the wiring 203_M is set such that anabsolute value of a difference between the voltage of the wiring 203_Min the M-th row and the voltage of each of the wirings 205_1 to 205 _(—)j is larger than or equal to a value of voltage necessary for bringingthe rectifier elements 213 (M, 1) to 213 (M, j) in the memory cells 200in the M-th row into conduction. Further, the voltage of the wirings203_1 to 203 _(—) i other than the wiring 203_M is set equivalent to thevoltage of each of the wirings 205_1 to 205 _(—) j, whereby the data canbe read only from the memory cells 200 in the M-th row.

For example, in the case where the transistors 212 (M, 1) to 212 (M, j)in the memory cells 200 in the M-th row are n-channel transistors, asillustrated in FIG. 3C, the voltage of the wiring 203_M is set to thevoltage VL; the voltage of the wirings 203_1 to 203 _(—) i other thanthe wiring 203_M is set to the voltage VH; and the voltage of thewirings 205_1 to 205 _(—) j is set to the voltage VH. Further, at thistime, the voltage of the wirings 202_1 to 202 _(—) i is set to thevoltage VL.

Resistance between the source and the drain of each of the transistors212 (M, 1) to 212 (M, j) in the memory cells 200 in the M-th row dependson the voltage of the gate of each of the transistors 212 (M, 1) to 212(M, j). Further, in accordance with current flowing between the sourceand the drain of each of the transistors 212 (M, 1) to 212 (M, j),whether or not the rectifier elements 213 (M, 1) to 213 (M, j) in thememory cells 200 in the M-th row are brought into conduction isdetermined and the voltage of the wirings 205_1 to 205 _(—) j is set.Therefore, when the voltage of the wirings 205_1 to 205 _(—) j at thistime is used as data, the data can be read from the memory cells 200 inthe M-th row. Moreover, during a period in which the transistors 211 (M,1) to 211 (M, j) are in an off state, the voltage of the gates of thetransistors 212 (M, 1) to 212 (M, j) is held for a predetermined period;thus, it is possible to read the voltage of the wirings 205_1 to 205_(—) j as the data from the memory cells 200 in the M-th row more thanonce.

In addition, when the above-described operation is performed for thememory cells 200 in each row, the data can be read from all the memorycells (the memory cells 200 (1, 1) to 200 (i, j)). The above isdescription of an example of a method for driving the semiconductormemory device illustrated in FIG. 3A.

The above is description of an example of the semiconductor memorydevice in Embodiment 2.

In this embodiment, the number of times written data can be read to onedata writing operation can be increased by setting the voltage of a gateof a second field-effect transistor, which is electrically connected toa source or a drain of a first field-effect transistor to a value basedon the voltage of a data signal.

Further, in this embodiment, by using a field-effect transistor with lowoff-state current as the first field-effect transistor, a data retentionperiod can be lengthened, and the capacitance of a capacitor added tothe gate of the second field-effect transistor can be reduced or thecapacitor can be omitted. Thus, even when a refresh operation is needed,for example, an interval between refresh operations can be made 10 yearsor longer.

Moreover, in an example of the semiconductor device in this embodiment,a wiring electrically connected to a source or a drain of the firstfield-effect transistor and a wiring electrically connected to a firstcurrent terminal of a rectifier element are the same (common) wiring,whereby the number of wirings can be reduced and the area of thesemiconductor memory device can be made small.

Embodiment 3

In this embodiment, an example of a transistor including an oxidesemiconductor layer which can be used for a semiconductor device or asemiconductor memory device in the above embodiment is described.

Examples of structures of the transistors in this embodiment aredescribed with reference to FIGS. 4A to 4D. FIGS. 4A to 4D arecross-sectional schematic views each illustrating an example of astructure of a transistor in this embodiment. Note that the componentsillustrated in FIGS. 4A to 4D include those having sizes different fromthe actual sizes.

A transistor illustrated in FIG. 4A is one of top-gate transistors.

The transistor illustrated in FIG. 4A includes a semiconductor layer603_A, a conductive layer 605 a_A, a conductive layer 605 b_A, aninsulating layer 606_A, and a conductive layer 607_A.

The semiconductor layer 603_A includes a region 604 a_A and a region 604b_A that are regions which are separated from each other and to which adopant is added. A region between the region 604 a_A and the region 604b_A serves as a channel formation region. The semiconductor layer 603_Ais provided over an element formation layer 600_A, for example.

The conductive layer 605 a_A is provided over the semiconductor layer603_A and is electrically connected to the semiconductor layer 603_A.Further, a side surface of the conductive layer 605 a_A is tapered andthe conductive layer 605 a_A partly overlaps with the region 604 a_A;however, the present invention is not necessarily limited thereto. Whenthe conductive layer 605 a_A partly overlaps with the region 604 a_A,resistance between the conductive layer 605 a_A and the region 604 a_Acan be low. Further, an entire region of the semiconductor layer 603_A,which overlaps with the conductive layer 605 a_A may be the region 604a_A.

The conductive layer 605 b_A is provided over the semiconductor layer603_A and is electrically connected to the semiconductor layer 603_A.Further, a side surface of the conductive layer 605 b_A is tapered andthe conductive layer 605 b_A partly overlaps with the region 604 b_A;however, the present invention is not necessarily limited thereto. Whenthe conductive layer 605 b_A partly overlaps with the region 604 b_A,resistance between the conductive layer 605 b_A and the region 604 b_Acan be low. Further, an entire region of the semiconductor layer 603_A,which overlaps with the conductive layer 605 b_A may be the region 604b_A.

The insulating layer 606_A is provided over the semiconductor layer603_A, the conductive layer 605 a_A, and the conductive layer 605 b_A.

The conductive layer 607_A overlaps with the semiconductor layer 603_Awith the insulating layer 606_A interposed therebetween. A region of thesemiconductor layer 603_A, which overlaps with the conductive layer607_A with the insulating layer 606_A interposed therebetween serves asthe channel formation region.

A transistor illustrated in FIG. 4B has the structure illustrated inFIG. 4A to which an insulating layer 609 a_A and an insulating layer 609b_A are added; moreover, the semiconductor layer 603_A includes, betweenthe region 604 a_A and the region 604 b_A, a region 608 a_A and a region608 b_A that are regions which are separated from each other and towhich a dopant is added.

The insulating layer 609 a_A is provided over the insulating layer 606_Aand is in contact with one of a pair of side surfaces of the conductivelayer 607_A which face each other.

The insulating layer 609 b_A is provided over the insulating layer 606_Aand is in contact with the other of the pair of side surfaces of theconductive layer 607_A which face each other.

The region 608 a_A overlaps with the insulating layer 609 a_A with theinsulating layer 606_A interposed therebetween. Further, theconcentration of the dopant of the region 608 a_A may be lower than thatof the dopant of the region 604 a_A and the region 604 b_A. In thatcase, the region 608 a_A is also referred to as a low concentrationregion.

The region 608 b_A overlaps with the insulating layer 609 b_A with theinsulating layer 606_A interposed therebetween. Further, theconcentration of the dopant of the region 608 b_A may be lower than thatof the dopant of the region 604 a_A and the region 604 b_A. In thatcase, the region 608 b_A is also referred to as a low concentrationregion. Further, in that case, the region 604 a_A and the region 604 b_Amay be referred to as high concentration regions.

When the region 608 a_A and the region 608 b_A are provided, localelectric field concentration on the transistor can be suppressed and thereliability of the transistor can be increased even when the area of thetransistor is small.

A transistor illustrated in FIG. 4C is one of top-gate transistors.

The transistor illustrated in FIG. 4C includes a semiconductor layer603_B, a conductive layer 605 a_B, a conductive layer 605 b_B, aninsulating layer 606_B, and a conductive layer 607_B.

The conductive layer 605 a_B is provided over an element formation layer600_B. Further, a side surface of the conductive layer 605 a_B istapered.

The conductive layer 605 b_B is provided over the element formationlayer 600_B. Further, a side surface of the conductive layer 605 b_B istapered.

The semiconductor layer 603_B includes a region 604 a_B and a region 604b_B that are regions which are separated from each other and to which adopant is added. A region between the region 604 a_B and the region 604b_B serves as a channel formation region. The semiconductor layer 603_Bis provided over the conductive layer 605 a_B, the conductive layer 605b_B, and the element formation layer 600_B, for example.

The region 604 a_B is electrically connected to the conductive layer 605a_B.

The region 604 b_B is electrically connected to the conductive layer 605b_B.

The insulating layer 606_B is provided over the semiconductor layer603_B.

The conductive layer 607_B overlaps with the semiconductor layer 603_Bwith the insulating layer 606_B interposed therebetween. A region of thesemiconductor layer 603_B, which overlaps with the conductive layer607_B with the insulating layer 606_B interposed therebetween serves asthe channel formation region.

A transistor illustrated in FIG. 4D has the structure illustrated inFIG. 4C to which an insulating layer 609 a_B and an insulating layer 609b_B are added; moreover, the semiconductor layer 603_B includes, betweenthe region 604 a_B and the region 604 b_B, a region 608 a_B and a region608 b_B that are regions which are separated from each other and towhich a dopant is added.

The insulating layer 609 a_B is provided over the insulating layer 606_Band is in contact with one of a pair of side surfaces of the conductivelayer 607_B which face each other.

The insulating layer 609 b_B is provided over the insulating layer 606_Band is in contact with the other of the pair of side surfaces of theconductive layer 607_B which face each other.

The region 608 a_B overlaps with the insulating layer 609 a_B with theinsulating layer 606_B interposed therebetween. Further, theconcentration of the dopant of the region 608 a_B may be lower than thatof the dopant of the region 604 a_B and the region 604 b_B. In thatcase, the region 608 a_B is also referred to as a low concentrationregion.

The region 608 b_B overlaps with the insulating layer 609 b_B with theinsulating layer 606_B interposed therebetween. Further, theconcentration of the dopant of the region 608 b_B may be lower than thatof the dopant of the region 604 a_B and the region 604 b_B. In thatcase, the region 608 b_B is also referred to as a low concentrationregion. Further, in that case, the region 604 a_B and the region 604 b_Bmay be referred to as high concentration regions.

When the region 608 a_B and the region 608 b_B are provided, localelectric field concentration on the transistor can be suppressed and thereliability of the transistor can be increased.

Next, the components illustrated in FIGS. 4A to 4D are described.

As the element formation layer 600_A and the element formation layer600_B, insulating layers, substrates having insulating surfaces, or thelike can be used, for example. Further, layers over which elements areformed in advance can be used as the element formation layer 600_A andthe element formation layer 600_B.

The semiconductor layer 603_A and the semiconductor layer 603_B serve aschannel formation layers of the transistors. As the semiconductor layer603_A and the semiconductor layer 603_B, oxide semiconductor layers canbe used.

An oxide semiconductor to be used preferably contains at least indium(In) or zinc (Zn). In particular, In and Zn are preferably contained. Asa stabilizer for reducing variation in electric characteristics of atransistor including the oxide semiconductor, it is preferable thatgallium (Ga) be additionally contained. Tin (Sn) is preferably containedas a stabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium(Lu) may be contained.

For the oxide semiconductor layer, an oxide semiconductor containingfour-component metal oxide, three-component metal oxide, two-componentmetal oxide, or the like can be used.

For example, as the four-component metal oxide, an In—Sn—Ga—Zn-basedoxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, anIn—Hf—Al—Zn-based oxide, or the like can be used.

For example, as the three-component metal oxide, an In—Ga—Zn-based oxide(also referred to as IGZO), an In—Sn—Zn-based oxide, an In—Al—Zn-basedoxide, Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-basedoxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, anIn—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide,an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-basedoxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, anIn—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide,an In—Yb—Zn-based oxide, an In—Lu—Zn-based oxide, or the like can beused.

For example, as the two-component metal oxide, an In—Zn-based oxide, aSn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, aSn—Mg-based oxide, an In—Mg-based oxide, an In—Sn-based oxide, anIn—Ga-based oxide, or the like can be used.

Further, as the semiconductor layer 603_A and the semiconductor layer603_B, for example, a layer of indium oxide, tin oxide, zinc oxide, orthe like can be used. The metal oxide which can be used for the oxidesemiconductor may contain silicon oxide. The metal oxide which can beused for the oxide semiconductor may contain nitrogen.

In the case where an In—Zn—O-based metal oxide is used, for example, anoxide target having the following composition ratios can be used forformation of an In—Zn—O-based metal oxide semiconductor layer:In:Zn=50:1 to 1:2 (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferablyIn:Zn=20:1 to 1:1 (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), morepreferably In:Zn=15:1 to 1.5:1 (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio).For example, when the atomic ratio of the target used for the formationof the In—Zn—O-based oxide semiconductor is expressed by In:Zn:O=P:Q:R,R>1.5P+Q. The increase in the In content can make the mobility of thetransistor higher.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxidecontaining In, Ga, and Zn as its main component, in which there is noparticular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxidemay contain a metal element other than In, Ga, and Zn.

As the semiconductor layer 603_A and the semiconductor layer 603_B, alayer containing a material represented by InLO₃(ZnO)_(m) (m>0, m is notan integer) can be used. Here, L in InLO₃(ZnO)_(m) represents one ormore metal elements selected from Ga, Al, Fe, Mn, and Co. Alternatively,as the oxide semiconductor, a material represented by In₃SnO₅(ZnO)_(n)(n>0, n is an integer) may be used.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or anyof oxides whose composition is in the neighborhood of the abovecompositions can be used. Alternatively, an In—Sn—Zn-based oxide with anatomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3(=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or any of oxides whosecomposition is in the neighborhood of the above compositions may beused.

However, without limitation to the materials given above, a materialwith an appropriate composition may be used depending on neededsemiconductor characteristics (e.g., mobility, threshold voltage, andvariation). In order to obtain the needed semiconductor characteristics,it is preferable that the carrier density, the impurity concentration,the defect density, the atomic ratio between a metal element and oxygen,the interatomic distance, the density, and the like be set toappropriate values.

For example, high mobility can be obtained relatively easily in the caseof using an In—Sn—Zn-based oxide. However, mobility can be increased byreducing the defect density in a bulk also in the case of using anIn—Ga—Zn-based oxide.

Note that for example, the expression “the composition of an oxideincluding In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide including In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy the following relationship: (a−A)²+(b−B)²+(c−C)²≦r², and rmay be 0.05, for example. The same applies to other oxides.

The oxide semiconductor may be either single crystal ornon-single-crystal. In the latter case, the oxide semiconductor may beeither amorphous or polycrystalline. Further, the oxide semiconductormay have either an amorphous structure including a portion havingcrystallinity or a non-amorphous structure.

In an oxide semiconductor in an amorphous state, a flat surface can beobtained relatively easily, so that when a transistor is manufacturedwith the use of the oxide semiconductor, interface scattering can bereduced, and relatively high mobility can be obtained relatively easily.

In an oxide semiconductor having crystallinity, defects in the bulk canbe further reduced and when a surface flatness is improved, mobilityhigher than that of an oxide semiconductor in an amorphous state can beobtained. In order to improve the surface flatness, the oxidesemiconductor is preferably formed over a flat surface. Specifically,the oxide semiconductor may be formed over a surface with the averagesurface roughness (R_(a)) of 1 nm or less, preferably 0.3 nm or less,further preferably 0.1 nm or less.

Note that the average surface roughness (R_(a)) is a value obtained byexpanding, into three dimensions, center line average roughness that isdefined by JIS B 0601 so as to be able to apply it to a plane. The R_(a)can be expressed as an “average value of the absolute values ofdeviations from a reference surface to a designated surface” and isdefined by the following formula.

$\begin{matrix}{{Ra} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f( {x,y} )} - Z_{0}}}{\mathbb{d}x}{\mathbb{d}y}}}}}} & \lbrack {{Formula}\mspace{14mu} 1} \rbrack\end{matrix}$

In the above formula, S₀ represents the area of a measurement plane (arectangular region which is defined by four points represented bycoordinates (x₁, y₁), (x₁, y₂), (x₂, y₁) and (x₂, y₂)), and Z₀represents an average height of the measurement plane. R_(a) can bemeasured using an atomic force microscope (AFM). The measurement planeis a plane where all the measurement data are shown, and the measurementdata consists of three parameters (X, Y, Z) and is represented by Z=F(X, Y). The range of X (and Y) is from 0 to X_(max) (and Y_(max)), andthe range of Z is from Z_(min) to Z_(max).

At least regions of the semiconductor layer 603_A and the semiconductorlayer 603_B, in which the channels are formed may be non-single-crystaland include a phase which has a triangular, hexagonal, regulartriangular, or regular hexagonal atomic arrangement when seen from thedirection perpendicular to the a-b plane and in which metal atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis direction or metal atoms and oxygen atoms are arranged ina layered manner when seen from the direction perpendicular to thec-axis direction.

A dopant imparting one conductivity type is added to the region 604 a_Aand the region 604 a_B, and the region 604 a_A and the region 604 a_Bserve as one of a source and a drain of a transistor. Note that a regionserving as a source of a transistor is also referred to as a sourceregion, and a region serving as a drain of a transistor is also referredto as a drain region.

A dopant imparting one conductivity type is added to the region 604 b_Aand the region 604 b_B, and the region 604 b_A and the region 604 b_Bserve as the other of the source and the drain of the transistor.

The region 608 a_A, the region 608 b_A, the region 608 a_B, and theregion 608 b_B may have lower resistance than the channel formationregions, and have higher resistance than the region 604 a_A, the region604 b_A, the region 604 a_B, and the region 604 b_B. Note that theregion 608 a_A, the region 608 b_A, the region 608 a_B, and the region608 b_B are also referred to as low-resistance regions.

As the dopants contained in the region 604 a_A, the region 604 b_A, theregion 608 a_A, the region 608 b_A, the region 604 a_B, the region 604b_B, the region 608 a_B, and the region 608 b_B, for example, one ormore elements selected from elements belonging to Group 15 of theperiodic table (e.g., one or more of nitrogen, phosphorus, and arsenic)and rare gas elements (e.g., one or more of helium, argon, and xenon)can be given.

The concentration of the dopants contained in the region 604 a_A, theregion 604 b_A, the region 604 a_B, and the region 604 b_B is preferably5×10¹⁹ cm⁻³ or higher, for example. For example, the region 604 a_A, theregion 604 b_A, the region 604 a_B, and the region 604 b_B may containnitrogen at a concentration of higher than or equal to 1×10²⁰ cm⁻³ andlower than 7 atomic %.

The concentration of the dopants contained in the region 608 a_A, theregion 608 b_A, the region 608 a_B, and the region 608 b_B is preferablyhigher than or equal to 5×10¹⁸ cm⁻³ and lower than 5×10¹⁹ cm⁻³, forexample.

In addition, the region 604 a_A, the region 604 b_A, the region 608 a_A,the region 608 b_A, the region 604 a_B, the region 604 b_B, the region608 a_B, and the region 608 b_B may have lower crystallinity than thechannel formation regions.

Further, the region 604 a_A, the region 604 b_A, the region 604 a_B, andthe region 604 b_B may contain a crystal having a wurtzite structure.

Further, the region 608 a_A, the region 608 b_A, the region 608 a_B, andthe region 608 b_B may contain a crystal having a wurtzite structure.

For example, the region 604 a_A, the region 604 b_A, the region 608 a_A,the region 608 b_A, the region 604 a_B, the region 604 b_B, the region608 a_B, and the region 608 b_B can contain a crystal having a wurtzitestructure by heat treatment after the addition of the dopants.

When the region to which the dopant is added contains a crystal having awurtzite structure, resistance between the channel formation region andthe source or drain of the transistor can be low.

The conductive layer 605 a_A, the conductive layer 605 a_B, theconductive layer 605 b_A, and the conductive layer 605 b_B serve as thesource or the drain of the transistor. Note that a layer serving as asource of a transistor is also referred to as a source electrode or asource wiring, and a layer serving as a drain of a transistor is alsoreferred to as a drain electrode or a drain wiring.

Each of the conductive layer 605 a_A, the conductive layer 605 a_B, theconductive layer 605 b_A, and the conductive layer 605 b_B can be, forexample, a layer of a metal material such as aluminum, chromium, copper,tantalum, titanium, molybdenum, or tungsten; or an alloy material whichcontains any of the above metal materials as a main component. As alayer of an alloy material, a layer of a Cu—Mg—Al alloy material can beused, for example.

Further, each of the conductive layer 605 a_A, the conductive layer 605a_B, the conductive layer 605 b_A, and the conductive layer 605 b_B canbe a layer containing conductive metal oxide. Note that silicon oxidemay be contained in conductive metal oxide that can be used for theconductive layer 605 a_A, the conductive layer 605 a_B, the conductivelayer 605 b_A, and the conductive layer 605 b_B.

Further, each of the conductive layer 605 a_A, the conductive layer 605a_B, the conductive layer 605 b_A, and the conductive layer 605 b_B canbe formed by stacking layers formed using materials that can be used forthe conductive layer 605 a_A, the conductive layer 605 a_B, theconductive layer 605 b_A, and the conductive layer 605 b_B. For example,when each of the conductive layer 605 a_A, the conductive layer 605 a_B,the conductive layer 605 b_A, and the conductive layer 605 b_B is formedby stacking a layer of copper over a layer of a Cu—Mg—Al alloy material,the conductive layer 605 a_A, the conductive layer 605 a_B, theconductive layer 605 b_A, and the conductive layer 605 b_B can have highadhesiveness with a layer which is in contact therewith.

As the insulating layer 606_A and the insulating layer 606_B, a siliconoxide layer, a silicon nitride layer, a silicon oxynitride layer, asilicon nitride oxide layer, an aluminum oxide layer, an aluminumnitride layer, an aluminum oxynitride layer, an aluminum nitride oxidelayer, or a hafnium oxide layer can be used, for example. Further, eachof the insulating layer 606_A and the insulating layer 606_B can beformed by stacking layers formed using materials that can be used forthe insulating layer 606_A and the insulating layer 606_B.

Alternatively, as each of the insulating layer 606_A and the insulatinglayer 606_B, an insulating layer of a material containing an elementthat belongs to Group 13 of the periodic table and oxygen can be used,for example.

Examples of the material containing an element that belongs to Group 13and oxygen include gallium oxide, aluminum oxide, aluminum galliumoxide, and gallium aluminum oxide. Note that aluminum gallium oxide is asubstance in which the amount of aluminum is larger than that of galliumin atomic percent, and gallium aluminum oxide is a substance in whichthe amount of gallium is larger than or equal to that of aluminum inatomic percent.

The conductive layer 607_A and the conductive layer 607_B serve as gatesof the transistors. Note that such a conductive layer serving as a gateof the transistor is also referred to as a gate electrode or a gatewiring.

Each of the conductive layer 607_A and the conductive layer 607_B canbe, for example, a layer of a metal material such as aluminum, chromium,copper, tantalum, titanium, molybdenum, or tungsten; or an alloymaterial which contains any of the above metal materials as a maincomponent. Further, each of the conductive layer 607_A and theconductive layer 607_B can be formed by stacking layers formed usingmaterials that can be used for the conductive layer 607_A and theconductive layer 607_B.

Further, each of the conductive layer 607_A and the conductive layer607_B can be a layer containing conductive metal oxide. Note thatsilicon oxide may be contained in conductive metal oxide that can beused for the conductive layer 607_A and the conductive layer 607_B.

Further, a layer of an In—Ga—Zn—O—N-based material can be used as theconductive layer 607_A and the conductive layer 607_B. A layer of anIn—Ga—Zn—O—N-based material has high conductivity and is thus suitablefor the conductive layer 607_A and the conductive layer 607_B.

As the insulating layer 609 a_A, the insulating layer 609 b_A, theinsulating layer 609 a_B, and the insulating layer 609 b_B, for example,a layer of a material that can be used for the insulating layer 606_Aand the insulating layer 606_B can be used. Further, each of theinsulating layer 609 a_A, the insulating layer 609 b_A, the insulatinglayer 609 a_B, and the insulating layer 609 b_B can be formed bystacking layers formed using materials that can be used for theinsulating layer 609 a_A, the insulating layer 609 b_A, the insulatinglayer 609 a_B, and the insulating layer 609 b_B.

Note that an insulating layer may be further provided over theinsulating layer 606_A with the conductive layer 607_A interposedtherebetween or over the insulating layer 606_B with the conductivelayer 607_B interposed therebetween.

Note that it is possible to suppress incidence of light on thesemiconductor layer when the transistor in this embodiment has astructure in which the entire semiconductor layer overlaps with theconductive layer serving as the gate electrode. In this case, the regionto which the dopant is added is not necessarily provided in thesemiconductor layer.

The above is description of a structure example of each of thetransistors illustrated in FIGS. 4A to 4D.

Next, as an example of a method for manufacturing the transistor in thisembodiment, an example of a method for manufacturing the transistorillustrated in FIG. 4A will be described with reference to FIGS. 5A to5E. FIGS. 5A to 5E are cross-sectional schematic views illustrating anexample of a method for manufacturing the transistor in this embodiment.

First, as illustrated in FIG. 5A, the semiconductor layer 603_A isformed over the element formation layer 600_A.

An example of a method for forming an oxide semiconductor layerincluding CAAC as an example of the semiconductor layer 603_A isdescribed below.

An example of the method for forming an oxide semiconductor layerincluding CAAC includes a step of forming a semiconductor film over theelement formation layer 600_A. Note that in an example of a method forforming the semiconductor layer 603_A, one of or both a step ofperforming heat treatment once or more than once and a step of removingpart of the semiconductor film may be included. In that case, a timingof the step of removing part of the semiconductor film is notparticularly limited as long as the step is performed after formation ofthe semiconductor film before formation of the conductive layer 605 a_Aand the conductive layer 605 b_A. Further, a timing of the step ofperforming the heat treatment is not particularly limited as long as thestep is performed after formation of the semiconductor film.

In the step of forming the semiconductor film over the element formationlayer 600_A, for example, the semiconductor film is formed by formationof a film of a material that can be used for the semiconductor layer603_A by sputtering. At this time, the temperature of the elementformation layer over which the semiconductor film is formed is set tohigher than or equal to 100° C. and lower than or equal to 500° C.,preferably higher than or equal to 200° C. and lower than or equal to350° C. With a high temperature of the element formation layer overwhich the semiconductor film is formed, the semiconductor film caninclude a phase which has a triangular, hexagonal, regular triangular,or regular hexagonal atomic arrangement when seen from the directionperpendicular to the a-b plane and in which metal atoms are arranged ina layered manner when seen from the direction perpendicular to thec-axis direction or metal atoms and oxygen atoms are arranged in alayered manner when seen from the direction perpendicular to the c-axisdirection.

Further, in the case where the semiconductor layer 603_A is formed withthe use of an In—Sn—Zn-based oxide (ITZO), an oxide target in which anatomic ratio of In, Sn, and Zn is 1:2:2, 2:1:3, 1:1:1, 20:45:35, or thelike is used.

In the step of performing the heat treatment, heat treatment (alsoreferred to as heat treatment A) is performed at higher than or equal to400° C. and lower than or equal to 750° C., or higher than or equal to400° C. and lower than the strain point of the substrate, for example.Note that a timing of the heat treatment A is not particularly limitedas long as the heat treatment is performed after formation of thesemiconductor film.

By the heat treatment A, the crystallinity of the semiconductor layer603_A can be increased.

Note that a heat treatment apparatus for the heat treatment A can be anelectric furnace or an apparatus for heating an object by heatconduction or heat radiation from a heater such as a resistance heater.For example, an RTA (rapid thermal annealing) apparatus such as a GRTA(gas rapid thermal annealing) apparatus, or an LRTA (lamp rapid thermalannealing) apparatus can be used. An LRTA apparatus is an apparatus forheating an object by radiation of light (an electromagnetic wave)emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenonarc lamp, a carbon arc lamp, a high pressure sodium lamp, or a highpressure mercury lamp. A GRTA apparatus is an apparatus with which heattreatment is performed using a high-temperature gas. As thehigh-temperature gas, for example, a rare gas or an inert gas (e.g.,nitrogen) which does not react with an object by heat treatment can beused.

After the heat treatment A, a high-purity oxygen gas, a high-purity N₂Ogas, or ultra-dry air (with a dew point of −40° C. or lower, preferably−60° C. or lower) may be introduced into the furnace that has been usedin the heat treatment A while the heating temperature is maintained ordecreased. In that case, it is preferable that water, hydrogen, and thelike be not contained in the oxygen gas or the N₂O gas. The purity ofthe oxygen gas or the N₂O gas which is introduced into the heattreatment apparatus is preferably 6N or higher, more preferably 7N orhigher. That is, the impurity concentration in the oxygen gas or the N₂Ogas is 1 ppm or lower, preferably 0.1 ppm or lower. By the action of theoxygen gas or the N₂O gas, oxygen is supplied to the semiconductor filmor the semiconductor layer 603_A, so that defects caused by oxygendeficiency in the semiconductor film or the semiconductor layer 603_Acan be reduced.

Then, as illustrated in FIG. 5B, a first conductive film is formed overpart of the semiconductor layer 603_A and is partly etched, so that theconductive layer 605 a_A and the conductive layer 605 b_A are formed.

For example, the first conductive film can be formed by formation of afilm formed using a material that can be used for the conductive layer605 a_A and the conductive layer 605 b_A by sputtering or the like.Alternatively, the first conductive film can be formed by stacking filmsformed using materials that can be used for the conductive layer 605 a_Aand the conductive layer 605 b_A.

In the case where a film is partly etched in the example of the methodfor manufacturing a transistor in this embodiment as in formation of theconductive layer 605 a_A and the conductive layer 605 b_A, the film maybe etched in such a manner that a resist mask is formed over part of thefilm through photolithography and is used, for example. Note that inthat case, the resist mask is preferably removed after the etching. Inaddition, the resist mask may be formed using an exposure mask having aplurality of regions with different transmittances (such an exposuremask is also referred to as a multi-tone mask). With the multi-tonemask, a resist mask having a plurality of regions with differentthicknesses can be formed, so that the number of resist masks used forthe formation of the transistor can be reduced.

Next, as illustrated in FIG. 5C, the insulating layer 606_A is formed byformation of a first insulating film over the semiconductor layer 603_A,the conductive layer 605 a_A, and the conductive layer 605 b_A. Thefirst insulating film can be formed by stacking films formed usingmaterials that can be used for the insulating layer 606_A.

For example, the first insulating film can be formed by formation of afilm formed using a material that can be used for the insulating layer606_A by sputtering, plasma-enhanced CVD, or the like. Further, when thefilm formed using a material that can be used for the insulating layer606_A is formed by high-density plasma-enhanced CVD (e.g., high-densityplasma-enhanced CVD using microwaves (e.g., microwaves with a frequencyof 2.45 GHz)), the insulating layer 606_A can be dense and can havehigher breakdown voltage.

Then, as illustrated in FIG. 5D, a second conductive film is formed overthe insulating layer 606_A and is partly etched, so that the conductivelayer 607_A is formed.

For example, the second conductive film can be formed by formation of afilm formed using a material that can be used for the conductive layer607_A by sputtering. Alternatively, the second conductive film can beformed by stacking films formed using materials that can be used for thesecond conductive film.

Note that when a high-purity gas from which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed is used as asputtering gas, for example, the impurity concentration in the film canbe lowered.

Note that heat treatment (also referred to as heat treatment B) may beperformed in a preheating chamber of a sputtering apparatus before thefilm is formed by sputtering. By the heat treatment B, an impurity suchas hydrogen or moisture can be eliminated.

Before the film is formed by sputtering, for example, treatment in whichvoltage is applied not to a target side but to a substrate side in anargon, nitrogen, helium, or oxygen atmosphere with the use of an RFpower and plasma is generated so that a surface on which the film isformed is modified (such treatment is also referred to as reversesputtering) may be performed. By reverse sputtering, powdery substances(also referred to as particles or dust) that attach onto the surface onwhich the film is formed can be removed.

In the case where the film is formed by sputtering, moisture remainingin a deposition chamber for the film can be removed by an adsorptionvacuum pump or the like. A cryopump, an ion pump, a titanium sublimationpump, or the like can be used as the adsorption vacuum pump.Alternatively, moisture remaining in the deposition chamber can beremoved by a turbo-molecular pump provided with a cold trap.

Further, after the insulating layer 606_A is formed, heat treatment(also referred to as heat treatment C) may be performed in an inert gasatmosphere or an oxygen gas atmosphere. At this time, the heat treatmentC can be performed at higher than or equal to 200° C. and lower than orequal to 400° C., preferably higher than or equal to 250° C. and lowerthan or equal to 350° C., for example.

Then, as illustrated in FIG. 5E, a dopant is added to the semiconductorlayer 603_A through the insulating layer 606_A from a side on which theconductive layer 607_A is formed, so that the region 604 a_A and theregion 604 b_A are formed in a self-aligned manner.

For example, the dopant can be added with the use of an ion dopingapparatus or an ion implantation apparatus.

As the dopant to be added, for example, one or more elements selectedfrom elements belonging to Group 15 of the periodic table (e.g., one ormore of nitrogen, phosphorus, and arsenic) and rare gas elements (e.g.,one or more of helium, argon, and xenon) can be used.

As one of the methods for forming a region serving as a source region ora drain region by a self-aligned process in a transistor which includesan oxide semiconductor layer serving as a channel formation layer, amethod has been disclosed in which a surface of an oxide semiconductorlayer is exposed, argon plasma treatment is performed, and theresistivity of a region of the oxide semiconductor layer that is exposedto the plasma is reduced (S. Jeon et al., “180 nm Gate Length AmorphousInGaZnO Thin Film Transistor for High Density Image SensorApplications”, IEDM Tech. Dig., 2010, p. 504).

However, in the above manufacturing method, for exposure of part of theoxide semiconductor layer which is to be the region serving as thesource region or the drain region, an insulating layer serving as a gateinsulating layer needs to be partly removed after being formed.Therefore, at the time of removal of the insulating layer serving as thegate insulating layer, the oxide semiconductor layer of a lower layer isalso partly over-etched and the thickness of the part which is to be theregion serving as the source region or the drain region is reduced. As aresult, resistance of the part which is to be the region serving as thesource region or the drain region is increased, and poor characteristicsof a transistor due to overetching easily occur.

For miniaturization of the transistor, a dry etching method, which hashigh processing accuracy, needs to be employed. However, the aboveoveretching tends to easily occur in the case of employing a dry etchingmethod, in which selectivity of the insulating layer serving as the gateinsulating layer to the oxide semiconductor layer cannot be sufficientlysecured.

For example, when the oxide semiconductor layer has a sufficientthickness, overetching does not cause a problem; however, when a channellength is 200 nm or less, the thickness of part of the oxidesemiconductor layer which is to be the channel formation region needs tobe 20 nm or less, preferably 10 nm or less so that a short channeleffect is prevented. In the case where such a thin oxide semiconductorlayer is handled, overetching of the oxide semiconductor layer isunfavorable because such overetching causes an increase in resistance ofthe region serving as the source region or the drain region and poorcharacteristics of the transistor which are described above.

On the other hand, when the dopant is added to the oxide semiconductorlayer while the oxide semiconductor layer is not exposed and theinsulating layer as the gate insulating layer is not removed, as in oneembodiment of the present invention, overetching of the oxidesemiconductor layer can be prevented and excessive damage to the oxidesemiconductor layer can be reduced. In addition, an interface betweenthe oxide semiconductor layer and the insulating layer serving as thegate insulating layer is also kept clean. Thus, the characteristics andreliability of the transistor can be improved.

Note that heat treatment may be performed after the dopant is added tothe semiconductor layer 603_A.

The above is description of an example of a method for manufacturing thetransistor illustrated in FIG. 4A.

Note that although the example of the method for manufacturing thetransistor illustrated in FIG. 4A is described, this embodiment is notlimited to this example. For example, as for the components in FIGS. 4Bto 4D that have the same designations as the components in FIG. 4A andwhose functions are at least partly the same as those of the componentsin FIG. 4A, the description of the example of the method formanufacturing the transistor illustrated in FIG. 4A can be referred toas appropriate.

For example, in the case where the transistor illustrated in FIG. 4B ismanufactured, after the step illustrated in FIG. 5D, a second insulatingfilm is formed over the insulating layer 606_A and the conductive layer607_A and is partly etched, so that the insulating layer 609 a_A and theinsulating layer 609 b_A are formed; then, the dopant is added to thesemiconductor layer 603_A, so that the dopant is added to part of thesemiconductor layer 603_A through the insulating layer 609 a_A and theinsulating layer 609 b_A, and the region 608 a_A and the region 608 b_Aare formed. Note that the second insulating film may be formed bysequential formation of a plurality of insulating films.

As described above with reference to FIGS. 4A to 4D and FIGS. 5A to 5E,an example of the transistor in this embodiment includes a semiconductorlayer in which a channel is formed, a conductive layer which iselectrically connected to the semiconductor layer and serves as one of asource and a drain, a conductive layer which is electrically connectedto the semiconductor layer and serves as the other of the source and thedrain, an insulating layer serving as a gate insulating layer, and aconductive layer which overlaps with the semiconductor layer with theinsulating layer interposed therebetween and serves as a gate.

Since the transistor in this embodiment has low off-state current, byemploying the transistor as a transistor serving as a selectiontransistor in the memory cell in the memory circuit or semiconductormemory device of the above semiconductor device, a data retention periodcan be lengthened. Accordingly, the number of times of refreshoperations can be reduced, which leads to a reduction in powerconsumption.

Further, in this embodiment, the oxide semiconductor layer serving as achannel formation layer includes a region to which a dopant is added andwhich has lower resistance than the channel formation region and servesas the source region or the drain region, whereby resistance between thechannel formation region and the source or drain of the transistor canbe low even when the area of the transistor is small.

Further, in this embodiment, a structure is employed in which theconductive layer serving as the gate and the conductive layer serving asthe source or the drain do not overlap with each other, wherebyparasitic capacitance between the gate and the source or the drain ofthe transistor can be reduced. Thus, a reduction in operating speed canbe suppressed even when the area of the transistor is made small.

Further, in this embodiment, deterioration of the transistor due tolight can be suppressed by employing a transistor including an oxidesemiconductor layer in which at least a channel formation region isnon-single-crystal and includes a phase which has a triangular,hexagonal, regular triangular, or regular hexagonal atomic arrangementwhen seen from the direction perpendicular to the a-b plane and in whichmetal atoms are arranged in a layered manner when seen from thedirection perpendicular to the c-axis direction or metal atoms andoxygen atoms are arranged in a layered manner when seen from thedirection perpendicular to the c-axis direction.

Embodiment 4

In this embodiment, an oxide including CAAC is described.

In a broad sense, an oxide including CAAC means a non-single-crystaloxide including a phase that has a triangular, hexagonal, regulartriangular, or regular hexagonal atomic arrangement when seen from thedirection perpendicular to the a-b plane and in which metal atoms arearranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis direction.

The CAAC is not a single crystal, but this does not mean that the CAACis composed of only an amorphous component. Although the CAAC includes acrystallized portion (crystalline portion), a boundary between onecrystalline portion and another crystalline portion is not clear in somecases.

In the case where oxygen is included in the CAAC, nitrogen may besubstituted for part of oxygen included in the CAAC. The c-axes ofindividual crystalline portions included in the CAAC may be aligned inone direction (e.g., a direction perpendicular to a surface of asubstrate over which the CAAC is formed or a surface of the CAAC).Alternatively, the normals of the a-b planes of the individualcrystalline portions included in the CAAC may be aligned in onedirection (e.g., a direction perpendicular to a surface of a substrateover which the CAAC is formed or a surface of the CAAC).

The CAAC becomes a conductor, a semiconductor, or an insulator dependingon its composition or the like. The CAAC transmits or does not transmitvisible light depending on its composition or the like.

An example of such a CAAC is a crystal which is formed into a film shapeand has a triangular or hexagonal atomic arrangement when observed fromthe direction perpendicular to a surface of the film or a surface of asupporting substrate, and in which metal atoms are arranged in a layeredmanner or metal atoms and oxygen atoms (or nitrogen atoms) are arrangedin a layered manner when a cross section of the film is observed.

An example of a crystal structure of the CAAC will be described indetail with reference to FIGS. 13A to 13E, FIGS. 14A to 14C, and FIGS.15A to 15C. In FIGS. 13A to 13E, FIGS. 14A to 14C, and FIGS. 15A to 15C,the vertical direction corresponds to the c-axis direction and a planeperpendicular to the c-axis direction corresponds to the a-b plane,unless otherwise specified. In the case where the expressions “an upperhalf” and “a lower half” are simply used, they refer to an upper halfabove the a-b plane and a lower half below the a-b plane (an upper halfand a lower half with respect to the a-b plane).

FIG. 13A illustrates a structure including one hexacoordinate In atomand six tetracoordinate oxygen (hereinafter referred to astetracoordinate O) atoms proximate to the In atom. Here, a structureincluding one metal atom and oxygen atoms proximate thereto is referredto as a small group. The structure in FIG. 13A is actually an octahedralstructure, but is illustrated as a planar structure for simplicity. Notethat three tetracoordinate O atoms exist in each of an upper half and alower half in FIG. 13A. In the small group illustrated in FIG. 13A,electric charge is 0.

FIG. 13B illustrates a structure including one pentacoordinate Ga atom,three tricoordinate oxygen (hereinafter referred to as tricoordinate O)atoms proximate to the Ga atom, and two tetracoordinate O atomsproximate to the Ga atom. All the tricoordinate O atoms exist on the a-bplane. One tetracoordinate O atom exists in each of an upper half and alower half in FIG. 13B. An In atom can also have the structureillustrated in FIG. 13B because an In atom can have five ligands. In thesmall group illustrated in FIG. 13B, electric charge is 0.

FIG. 13C illustrates a structure including one tetracoordinate Zn atomand four tetracoordinate O atoms proximate to the Zn atom. In FIG. 13C,one tetracoordinate O atom exists in an upper half and threetetracoordinate O atoms exist in a lower half. Alternatively, threetetracoordinate O atoms may exist in the upper half and onetetracoordinate O atom may exist in the lower half in FIG. 13C. In thesmall group illustrated in FIG. 13C, electric charge is 0.

FIG. 13D illustrates a structure including one hexacoordinate Sn atomand six tetracoordinate O atoms proximate to the Sn atom. In FIG. 13D,three tetracoordinate O atoms exist in each of an upper half and a lowerhalf. In the small group illustrated in FIG. 13D, electric charge is +1.

FIG. 13E illustrates a small group including two Zn atoms. In FIG. 13E,one tetracoordinate O atom exists in each of an upper half and a lowerhalf. In the small group illustrated in FIG. 13E, electric charge is −1.

Here, a plurality of small groups form a medium group, and a pluralityof medium groups form a large group (also referred to as a unit cell).

Now, a rule of bonding between the small groups will be described. Thethree O atoms in the upper half with respect to the hexacoordinate Inatom in FIG. 13A each have three proximate In atoms in the downwarddirection, and the three O atoms in the lower half each have threeproximate In atoms in the upward direction. The one O atom in the upperhalf with respect to the pentacoordinate Ga atom in FIG. 13B has oneproximate Ga atom in the downward direction, and the one O atom in thelower half has one proximate Ga atom in the upward direction. The one Oatom in the upper half with respect to the tetracoordinate Zn atom inFIG. 13C has one proximate Zn atom in the downward direction, and thethree O atoms in the lower half each have three proximate Zn atoms inthe upward direction. In this manner, the number of the tetracoordinateO atoms above the metal atom is equal to the number of the metal atomsproximate to and below each of the tetracoordinate O atoms. Similarly,the number of the tetracoordinate O atoms below the metal atom is equalto the number of the metal atoms proximate to and above each of thetetracoordinate O atoms. Since the coordination number of thetetracoordinate O atom is 4, the sum of the number of the metal atomsproximate to and below the O atom and the number of the metal atomsproximate to and above the O atom is 4. Accordingly, when the sum of thenumber of tetracoordinate O atoms above a metal atom and the number oftetracoordinate O atoms below another metal atom is 4, the two kinds ofsmall groups including the metal atoms can be bonded. For example, inthe case where the hexacoordinate metal (In or Sn) atom is bondedthrough three tetracoordinate O atoms in the lower half, it is bonded tothe pentacoordinate metal (Ga or In) atom or the tetracoordinate metal(Zn) atom.

A metal atom having the above coordination number is bonded to anothermetal atom having the above coordination number through atetracoordinate O atom in the c-axis direction. In addition to theabove, a medium group can be formed in a different manner by combining aplurality of small groups so that the total electric charge of thelayered structure is 0.

FIG. 14A illustrates a model of a medium group included in a layeredstructure of an In—Sn—Zn—O-based material. FIG. 14B illustrates a largegroup including three medium groups. FIG. 14C illustrates an atomicarrangement in the case where the layered structure in FIG. 14B isobserved from the c-axis direction.

In FIG. 14A, for simplicity, a tricoordinate O atom is omitted andtetracoordinate O atoms are shown by a circle; the number in the circleshows the number of tetracoordinate O atoms. For example, threetetracoordinate O atoms existing in each of an upper half and a lowerhalf with respect to a Sn atom are denoted by circled 3. Similarly, inFIG. 14A, one tetracoordinate O atom existing in each of an upper halfand a lower half with respect to an In atom is denoted by circled 1.FIG. 14A also illustrates a Zn atom proximate to one tetracoordinate Oatom in a lower half and three tetracoordinate O atoms in an upper half,and a Zn atom proximate to one tetracoordinate O atom in an upper halfand three tetracoordinate O atoms in a lower half.

In the medium group included in the layered structure of theIn—Sn—Zn—O-based material in FIG. 14A, in the order starting from thetop, a Sn atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to an In atom proximate to onetetracoordinate O atom in each of an upper half and a lower half, the Inatom is bonded to a Zn atom proximate to three tetracoordinate O atomsin an upper half, the Zn atom is bonded to an In atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to theZn atom, the In atom is bonded to a small group that includes two Znatoms and is proximate to one tetracoordinate O atom in an upper half,and the small group is bonded to a Sn atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to thesmall group. A plurality of such medium groups are bonded, so that alarge group is formed.

Here, electric charge for one bond of a tricoordinate O atom andelectric charge for one bond of a tetracoordinate O atom can be assumedto be −0.667 and −0.5, respectively. For example, electric charge of a(hexacoordinate or pentacoordinate) In atom, electric charge of a(tetracoordinate) Zn atom, and electric charge of a (pentacoordinate orhexacoordinate) Sn atom are +3, +2, and +4, respectively. Accordingly,electric charge in a small group including a Sn atom is +1. Therefore,electric charge of −1, which cancels +1, is needed to form a layeredstructure including a Sn atom. As a structure having electric charge of−1, the small group including two Zn atoms as illustrated in FIG. 13Ecan be given. For example, with one small group including two Zn atoms,electric charge of one small group including a Sn atom can be cancelled,so that the total electric charge of the layered structure can be 0.

When the large group illustrated in FIG. 14B is repeated, anIn—Sn—Zn—O-based crystal (In₂SnZn₃O₈) can be obtained. Note that alayered structure of the obtained In—Sn—Zn—O-based crystal can beexpressed as a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or anatural number). As larger m is, the crystallinity of theIn—Sn—Zn—O-based crystal is improved, which is preferable.

The above-described rule also applies to the following oxides: afour-component metal oxide, such as an In—Sn—Ga—Zn-based oxide; athree-component metal oxide, such as an In—Ga—Zn-based oxide (alsoreferred to as IGZO), an In—Al—Zn-based oxide, a Sn—Ga—Zn-based oxide,an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-basedoxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, anIn—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Pm—Zn-based oxide,an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-basedoxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, anIn—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide,an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; a two-componentmetal oxide, such as an In—Zn-based oxide, a Sn—Zn-based oxide, anAl—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, anIn—Mg-based oxide, or an In—Ga-based oxide; a single-component metaloxide, such as an In-based oxide, a Sn-based oxide, or a Zn-based oxide;and the like.

As an example, FIG. 15A illustrates a model of a medium group includedin a layered structure of an In—Ga—Zn—O-based material.

In the medium group included in the layered structure of theIn—Ga—Zn—O-based material in FIG. 15A, in the order starting from thetop, an In atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to a Zn atom proximate to onetetracoordinate O atom in an upper half, the Zn atom is bonded to a Gaatom proximate to one tetracoordinate O atom in each of an upper halfand a lower half through three tetracoordinate O atoms in a lower halfwith respect to the Zn atom, and the Ga atom is bonded to an In atomproximate to three tetracoordinate O atoms in each of an upper half anda lower half through one tetracoordinate O atom in a lower half withrespect to the Ga atom. A plurality of such medium groups are bonded, sothat a large group is formed.

FIG. 15B illustrates a large group including three medium groups. FIG.15C illustrates an atomic arrangement in the case where the layeredstructure in FIG. 15B is observed from the c-axis direction.

Here, since electric charge of a (hexacoordinate or pentacoordinate) Inatom, electric charge of a (tetracoordinate) Zn atom, and electriccharge of a (pentacoordinate) Ga atom are +3, +2, and +3, respectively,electric charge of a small group including any of an In atom, a Zn atom,and a Ga atom is 0. As a result, the total electric charge of a mediumgroup having a combination of such small groups is always 0.

In order to form the layered structure of the In—Ga—Zn—O-based material,a large group can be formed using not only the medium group illustratedin FIG. 15A but also a medium group in which the arrangement of the Inatom, the Ga atom, and the Zn atom is different from that in FIG. 15A.

Embodiment 5

In this embodiment, an example of a structure of a memory cell in thesemiconductor memory device in Embodiment 2 is described with referenceto FIG. 6. FIG. 6 is a cross-sectional schematic view illustrating anexample of a structure of the memory cell in this embodiment. An exampleof a structure of a memory cell that has the structure illustrated inFIGS. 3A to 3C is described here as an example. Note that the componentsillustrated in FIG. 6 include those having sizes different from theactual sizes.

The memory cell illustrated in FIG. 6 includes a semiconductor layer700, an insulating layer 705, conductive layers 706 a, 706 b, and 706 c,insulating layers 707 a, 707 b, 707 c, 707 d, 707 e, and 707 f,insulating layers 708 a, 708 b, 708 c, 708 d, 708 e, and 708 f, aninsulating layer 709, a semiconductor layer 710, conductive layers 712a, 712 b, and 712 c, an insulating layer 713, and a conductive layer714.

The semiconductor layer 700 includes regions 702, 703 a, 703 b, 704 a,and 704 b.

As the semiconductor layer 700, for example, a semiconductor substratecan be used. Alternatively, a semiconductor layer provided over adifferent substrate can be used as the semiconductor layer 700.

Note that in a region of the semiconductor layer 700 which is between aplurality of memory cells, an insulating separation region may beprovided.

The region 702 is a region to which a dopant imparting one of n-typeconductivity and p-type conductivity is added.

The region 703 a and the region 703 b are regions which are separatedfrom each other and to which a dopant imparting the other of the n-typeconductivity and the p-type conductivity is added. The region 703 a andthe region 703 b serve as a source region and a drain region of anoutput transistor in the memory cell.

Note that a diode in the memory cell is formed with the region 702 andthe region 703 a. The present invention is not limited thereto and adiode-connected transistor may be separately formed as a rectifierelement.

The region 704 a and the region 704 b are provided between the region703 a and the region 703 b so as to be separated from each other, and achannel formation region is formed between the region 704 a and theregion 704 b. The region 704 a is in contact with the region 703 a, andthe region 704 b is in contact with the region 703 b.

As in the region 703 a and the region 703 b, the region 704 a and theregion 704 b are regions to which a dopant imparting the other of then-type conductivity and the p-type conductivity is added.

Note that the concentration of the dopant of the region 704 a and theregion 704 b may be lower than the concentration of the dopant of theregion 703 a and the region 703 b. In that case, the region 704 a andthe region 704 b are also referred to as low concentration regions.Further, in that case, the region 703 a and the region 703 b may bereferred to as high concentration regions. The region 704 a and theregion 704 b may have a shallower depth than the region 703 a and theregion 703 b; however, the present invention is not limited thereto.

The insulating layer 705 is provided over the semiconductor layer 700.The insulating layer 705 serves as a gate insulating layer of the outputtransistor in the memory cell.

As the insulating layer 705, for example, a layer of a material such assilicon oxide, silicon nitride, silicon oxynitride, silicon nitrideoxide, aluminum oxide, aluminum nitride, aluminum oxynitride, aluminumnitride oxide, hafnium oxide, or an organic insulating material (e.g.,polyimide or acrylic) can be used. The insulating layer 705 may beformed by stacking layers formed using materials that can be used forthe insulating layer 705.

The conductive layer 706 a is provided over the insulating layer 705 andis electrically connected to the region 702 through a first openingportion penetrating the insulating layer 705.

The conductive layer 706 b overlaps with the semiconductor layer 700with the insulating layer 705 interposed therebetween. A region of thesemiconductor layer 700, which overlaps with the conductive layer 706 bserves as the channel formation region of the output transistor in thememory cell. The conductive layer 706 b serves as a gate of the outputtransistor in the memory cell.

The conductive layer 706 c is provided over the insulating layer 705 andis electrically connected to the region 703 b through a second openingportion penetrating the insulating layer 705.

The insulating layer 707 a is provided over the insulating layer 705 andis in contact with one of a pair of side surfaces of the conductivelayer 706 a which face each other.

The insulating layer 707 b is provided over the insulating layer 705 andis in contact with the other of the pair of side surfaces of theconductive layer 706 a which face each other.

The insulating layer 707 c is provided over the insulating layer 705 andis in contact with one of a pair of side surfaces of the conductivelayer 706 b which face each other. Further, the insulating layer 707 coverlaps with the region 704 a with the insulating layer 705 interposedtherebetween.

The insulating layer 707 d is provided over the insulating layer 705 andis in contact with the other of the pair of side surfaces of theconductive layer 706 b which face each other. Further, the insulatinglayer 707 d overlaps with the region 704 b with the insulating layer 705interposed therebetween.

The insulating layer 707 e is provided over the insulating layer 705 andis in contact with one of a pair of side surfaces of the conductivelayer 706 c which face each other.

The insulating layer 707 f is provided over the insulating layer 705 andis in contact with the other of the pair of side surfaces of theconductive layer 706 c which face each other.

The insulating layer 708 a is provided over the insulating layer 707 a.

The insulating layer 708 b is provided over the insulating layer 707 b.

The insulating layer 708 c is provided over the insulating layer 707 c.

The insulating layer 708 d is provided over the insulating layer 707 d.

The insulating layer 708 e is provided over the insulating layer 707 e.

The insulating layer 708 f is provided over the insulating layer 707 f.

The insulating layer 709 is provided over the insulating layer 705.

The semiconductor layer 710 is provided over the insulating layer 709.The semiconductor layer 710 includes a region 711 a and a region 711 b.

As the semiconductor layer 710, for example, a layer formed using amaterial that can be used for the semiconductor layer 603_A of thetransistor described in Embodiment 3 with reference to FIG. 4A can beused. The semiconductor layer 710 serves as a channel formation layer ofa transistor that serves as a selection transistor in the memory cell.

The region 711 a and the region 711 b are regions to which a dopantimparting n-type conductivity is added. For the region 711 a and theregion 711 b, a dopant which can be used as the dopant added to theregion 604 a_A and the region 604 b_A of the transistor that isdescribed in Embodiment 3 with reference to FIG. 4A can be used. Theconcentration of the dopant of the region 711 a and the region 711 b canbe set within the range of the concentration of the dopant which can beemployed for the region 604 a_A and the region 604 b_A.

The region 711 a and the region 711 b are provided so as to be separatedfrom each other. A channel formation region is formed between the region711 a and the region 711 b.

The conductive layer 712 a is electrically connected to the conductivelayer 706 a and the semiconductor layer 710. Further, part of theconductive layer 712 a overlaps with the region 711 a. The conductivelayer 712 a serves as one of a source and a drain of the transistor thatserves as the selection transistor in the memory cell.

The conductive layer 712 b is electrically connected to the conductivelayer 706 b and the semiconductor layer 710. Further, part of theconductive layer 712 b overlaps with the region 711 b. The conductivelayer 712 b serves as the other of the source and the drain of thetransistor that serves as the selection transistor in the memory cell.

The conductive layer 712 c is electrically connected to the conductivelayer 706 c.

As the conductive layers 712 a to 712 c, for example, a layer formedusing a material that can be used for the conductive layer 605 a_A andthe conductive layer 605 b_A of the transistor described in Embodiment 3with reference to FIG. 4A can be used.

The insulating layer 713 is provided over the semiconductor layer 710.The insulating layer 713 serves as a gate insulating layer of theselection transistor in the memory cell.

As the insulating layer 713, for example, a layer formed using amaterial that can be used for the insulating layer 606_A of thetransistor described in Embodiment 3 with reference to FIG. 4A can beused.

The conductive layer 714 overlaps with the semiconductor layer 710 withthe insulating layer 713 interposed therebetween. The conductive layer714 serves as a gate of the transistor which serves as the selectiontransistor in the memory cell.

As the conductive layer 714, for example, a layer formed using amaterial that can be used for the conductive layer 607_A of thetransistor described in Embodiment 3 with reference to FIG. 4A can beused.

The above is description of an example of a structure of the memory cellillustrated in FIG. 6.

In this embodiment, the area of a memory cell can be made small byemploying a stack of a plurality of transistors.

In addition, in this embodiment, a transistor serving as an outputtransistor in the memory cell includes a semiconductor layer whichcontains a semiconductor belonging to Group 14 (e.g., silicon) and hashigh current supply capability, and a transistor serving as a selectiontransistor in the memory cell includes an oxide semiconductor layer andhas low off-state current; accordingly, operating speed of the memorycell can be high and a data retention period can be lengthened. Thus,power consumption can be reduced.

Furthermore, in this embodiment, the oxide semiconductor layer which isincluded in the transistor serving as the selection transistor includesa region to which a dopant is added, whereby resistance between achannel formation region and a source or a drain can be low even whenthe area of the memory cell is small.

Embodiment 6

In this embodiment, the field-effect mobility of a transistor will bedescribed.

The actually measured field-effect mobility of an insulated gatetransistor can be lower than its original mobility because of a varietyof reasons; this phenomenon occurs not only in the case of using anoxide semiconductor. One of the reasons that reduce the mobility is adefect inside a semiconductor or a defect at an interface between thesemiconductor and an insulating film. When a Levinson model is used, thefield-effect mobility on the assumption that no defect exists inside thesemiconductor can be calculated theoretically.

Assuming that the original mobility and the measured field-effectmobility of a semiconductor are μ₀ and μ, respectively, and a potentialbarrier (such as a grain boundary) exists in the semiconductor, themeasured field-effect mobility can be expressed as the followingformula.

$\begin{matrix}{\mu = {\mu_{0}{\exp( {- \frac{E}{kT}} )}}} & \lbrack {{Formula}\mspace{14mu} 2} \rbrack\end{matrix}$

Here, E represents the height of the potential barrier, k represents theBoltzmann constant, and T represents the absolute temperature. When thepotential barrier is assumed to be attributed to a defect, the height ofthe potential barrier can be expressed as the following formulaaccording to the Levinson model.

$\begin{matrix}{E = {\frac{{\mathbb{e}}^{2}N^{2}}{8ɛ\; n} = \frac{{\mathbb{e}}^{3}N^{2}t}{8\; ɛ\; C_{ox}V_{g}}}} & \lbrack {{Formula}\mspace{14mu} 3} \rbrack\end{matrix}$

Here, e represents the elementary charge, N represents the averagedefect density per unit area in a channel, ∈ represents the dielectricconstant of the semiconductor, n represents the number of carriers perunit area in the channel, C_(ox), represents the capacitance per unitarea, V_(g) represents the gate voltage, and t represents the thicknessof the channel. In the case where the thickness of the semiconductorlayer is 30 nm or less, the thickness of the channel may be regarded asbeing the same as the thickness of the semiconductor layer. The draincurrent I_(d) in a linear region can be expressed as the followingformula.

$\begin{matrix}{I_{d} = {\frac{W\;\mu\; V_{g}V_{d}C_{ox}}{L}{\exp( {- \frac{E}{kT}} )}}} & \lbrack {{Formula}\mspace{14mu} 4} \rbrack\end{matrix}$

Here, L represents the channel length and W represents the channelwidth, and L and W are each 10 μm. In addition, V_(d) represents thedrain voltage. When dividing both sides of the above equation by V_(g)and then taking logarithms of both sides, the following formula can beobtained.

$\begin{matrix}\begin{matrix}{{\ln( \frac{I_{d}}{V_{g}} )} = {{\ln( \frac{W\;\mu\; V_{d}C_{ox}}{L} )} - \frac{E}{kT}}} \\{= {{\ln( \frac{W\;\mu\; V_{d}C_{ox}}{L} )} - \frac{{\mathbb{e}}^{3}N^{2}t}{8\;{kT}\; ɛ\; C_{ox}V_{g}}}}\end{matrix} & \lbrack {{Formula}\mspace{14mu} 5} \rbrack\end{matrix}$

The right side of Formula 5 is a function of V_(g). From the formula, itis found that the defect density N can be obtained from the slope of aline in which ln(I_(d)/V_(g)) is the ordinate and 1/V_(g) is theabscissa. That is, the defect density can be evaluated from theI_(d)-V_(g) characteristics of the transistor. The defect density N ofan oxide semiconductor in which the ratio of indium (In), tin (Sn), andzinc (Zn) is 1:1:1 is approximately 1×10¹²/cm².

On the basis of the defect density obtained in this manner, or the like,μ₀ can be calculated to be 120 cm²/Vs from Formula 2 and Formula 3. Themeasured mobility of an In—Sn—Zn-based oxide including a defect isapproximately 35 cm²/Vs. However, assuming that no defect exists insidethe semiconductor and at the interface between the semiconductor and aninsulating film, the mobility μ₀ of the oxide semiconductor is expectedto be 120 cm²/Vs.

Note that even when no defect exists inside a semiconductor, scatteringat an interface between a channel and a gate insulating layer affectsthe transport property of the transistor. In other words, the mobilityμ₁ at a position that is distance x away from the interface between thechannel and the gate insulating layer can be expressed as the followingformula.

$\begin{matrix}{\frac{1}{\mu_{1}} = {\frac{1}{\mu_{0}} + {\frac{D}{B}{\exp( {- \frac{x}{G}} )}}}} & \lbrack {{Formula}\mspace{14mu} 6} \rbrack\end{matrix}$

Here, D represents the electric field in the gate direction, and B and Gare constants. B and G can be obtained from actual measurement results;according to the above measurement results, B is 4.75×10⁷ cm/s and G is10 nm (the depth to which the influence of interface scatteringreaches). When D is increased (i.e., when the gate voltage isincreased), the second term of Formula 6 is increased and accordinglythe mobility μ₁ is decreased.

Calculation results of the mobility μ₂ of a transistor whose channelincludes an ideal oxide semiconductor without a defect inside thesemiconductor are shown in FIG. 16. For the calculation, devicesimulation software Sentaurus Device manufactured by Synopsys, Inc. wasused, and the band gap, the electron affinity, the relativepermittivity, and the thickness of the oxide semiconductor were assumedto be 2.8 eV, 4.7 eV, 15, and 15 nm, respectively. These values wereobtained by measurement of a thin film that was formed by sputtering.

Further, the work functions of a gate, a source, and a drain wereassumed to be 5.5 eV, 4.6 eV, and 4.6 eV, respectively. The thickness ofa gate insulating layer was assumed to be 100 nm, and the relativepermittivity thereof was assumed to be 4.1. The channel length and thechannel width were each assumed to be 10 μm, and the drain voltage V_(d)was assumed to be 0.1 V.

As shown in FIG. 16, the mobility has a peak of 100 cm²/Vs or more at agate voltage that is a little over 1 V and is decreased as the gatevoltage becomes higher because the influence of interface scattering isincreased. Note that in order to reduce interface scattering, it isdesirable that a surface of the semiconductor layer be flat at theatomic level (atomic layer flatness).

Calculation results of characteristics of minute transistors which aremanufactured using an oxide semiconductor having such a mobility areshown in FIGS. 17A to 17C, FIGS. 18A to 18C, and FIGS. 19A to 19C. FIGS.20A and 20B illustrate cross-sectional structures of the transistorsused for the calculation. The transistors illustrated in FIGS. 20A and20B each include a semiconductor region 903 a and a semiconductor region903 c which have n⁺-type conductivity in an oxide semiconductor layer.The resistivities of the semiconductor region 903 a and thesemiconductor region 903 c are 2×10⁻³ Ωcm.

The transistor illustrated in FIG. 20A is formed over a base insulatingfilm 901 and an embedded insulator 902 which is embedded in the baseinsulating film 901 and formed of aluminum oxide. The transistorincludes the semiconductor region 903 a, the semiconductor region 903 c,an intrinsic semiconductor region 903 b serving as a channel formationregion therebetween, and a gate electrode 905. The width of the gateelectrode 905 is 33 nm.

A gate insulating layer 904 is formed between the gate electrode 905 andthe semiconductor region 903 b. In addition, a sidewall insulator 906 aand a sidewall insulator 906 b are formed on both side surfaces of thegate electrode 905, and an insulator 907 is formed over the gateelectrode 905 so as to prevent a short circuit between the gateelectrode 905 and another wiring. The sidewall insulator has a width of5 nm. A source electrode 908 a and a drain electrode 908 b are providedin contact with the semiconductor region 903 a and the semiconductorregion 903 c, respectively. Note that the channel width of thistransistor is 40 nm.

The transistor of FIG. 20B is the same as the transistor of FIG. 20A inthat it is formed over the base insulating film 901 and the embeddedinsulator 902 formed of aluminum oxide and that it includes thesemiconductor region 903 a, the semiconductor region 903 c, theintrinsic semiconductor region 903 b provided therebetween, the gateelectrode 905 having a width of 33 nm, the gate insulating layer 904,the sidewall insulator 906 a, the sidewall insulator 906 b, theinsulator 907, the source electrode 908 a, and the drain electrode 908b.

The transistor illustrated in FIG. 20B is different from the transistorillustrated in FIG. 20A in the conductivity type of semiconductorregions under the sidewall insulator 906 a and the sidewall insulator906 b. In the transistor illustrated in FIG. 20A, the semiconductorregions under the sidewall insulator 906 a and the sidewall insulator906 b are part of the semiconductor region 903 a having n⁺-typeconductivity and part of the semiconductor region 903 c having n⁺-typeconductivity, whereas in the transistor illustrated in FIG. 20B, thesemiconductor regions under the sidewall insulator 906 a and thesidewall insulator 906 b are part of the intrinsic semiconductor region903 b. In other words, a region having a width of L_(off) which overlapswith neither the semiconductor region 903 a (the semiconductor region903 c) nor the gate electrode 905 is provided. This region is called anoffset region, and the width L_(off) is called an offset length. As isseen from the drawing, the offset length is equal to the width of thesidewall insulator 906 a (the sidewall insulator 906 b).

The other parameters used in calculation are as described above. For thecalculation, device simulation software Sentaurus Device manufactured bySynopsys, Inc. was used. FIGS. 17A to 17C show the gate voltage (V_(g):a potential difference between the gate and the source) dependence ofthe drain current (I_(d), a solid line) and the mobility (μ, a dottedline) of the transistor having the structure illustrated in FIG. 20A.The drain current I_(d) is obtained by calculation under the assumptionthat the drain voltage (a potential difference between the drain and thesource) is +1 V and the mobility μ is obtained by calculation under theassumption that the drain voltage is +0.1 V.

FIG. 17A shows the gate voltage dependence of the transistor in the casewhere the thickness of the gate insulating layer is 15 nm, FIG. 17Bshows that of the transistor in the case where the thickness of the gateinsulating layer is 10 nm, and FIG. 17C shows that of the transistor inthe case where the thickness of the gate insulating layer is 5 nm As thegate insulating layer is thinner, the drain current I_(d) (off-statecurrent) particularly in an off state is significantly decreased. Incontrast, there is no noticeable change in the peak value of themobility μ and the drain current I_(d) in an on state (on-statecurrent). The graphs show that the drain current exceeds 10 μA at a gatevoltage of around 1 V.

FIGS. 18A to 18C show the gate voltage V_(g) dependence of the draincurrent I_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor having the structure illustrated in FIG. 20B where the offsetlength L_(off) is 5 nm. The drain current I_(d) is obtained bycalculation under the assumption that the drain voltage is +1 V and themobility μ is obtained by calculation under the assumption that thedrain voltage is +0.1 V. FIG. 18A shows the gate voltage dependence ofthe transistor in the case where the thickness of the gate insulatinglayer is 15 nm, FIG. 18B shows that of the transistor in the case wherethe thickness of the gate insulating layer is 10 nm, and FIG. 18C showsthat of the transistor in the case where the thickness of the gateinsulating layer is 5 nm.

Further, FIGS. 19A to 19C show the gate voltage dependence of the draincurrent I_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor having the structure illustrated in FIG. 20B where the offsetlength L_(off) is 15 nm. The drain current I_(d) is obtained bycalculation under the assumption that the drain voltage is +1 V and themobility μ is obtained by calculation under the assumption that thedrain voltage is +0.1 V. FIG. 19A shows the gate voltage dependence ofthe transistor in the case where the thickness of the gate insulatinglayer is 15 nm, FIG. 19B shows that of the transistor in the case wherethe thickness of the gate insulating layer is 10 nm, and FIG. 19C showsthat of the transistor in the case where the thickness of the gateinsulating layer is 5 nm.

In either of the structures, as the gate insulating layer is thinner,the off-state current is significantly decreased, whereas no noticeablechange arises in the peak value of the mobility μ and the on-statecurrent.

Note that the peak of the mobility μ is approximately 80 cm²/Vs in FIGS.17A to 17C, approximately 60 cm²/Vs in FIGS. 18A to 18C, andapproximately 40 cm²/Vs in FIGS. 19A to 19C; thus, the peak of themobility μ is decreased as the offset length L_(off) is increased.Further, the same applies to the off-state current. The on-state currentis also decreased as the offset length L_(off) is increased; however,the decrease in the on-state current is much more gradual than thedecrease in the off-state current. Further, the graphs show that ineither of the structures, the drain current exceeds 10 μA at a gatevoltage of around 1 V.

Embodiment 7

In this embodiment, an example of a structure of a semiconductor memorydevice is described.

First, an example of a structure of a semiconductor memory device inthis embodiment is described with reference to FIG. 7. FIG. 7 is a blockdiagram illustrating an example of a structure of the semiconductormemory device in this embodiment.

The semiconductor memory device illustrated in FIG. 7 includes a memorycell array (also referred to as MCA) 812 including a plurality of memorycells (also referred to as MC) 811, a first driver circuit (alsoreferred to as IDRV) 813_1, a second driver circuit (also referred to asJDRV) 813_2, and a drive control circuit (also referred to as DCTL)813_3.

As the structure of the memory cell array, the structure of the memorycell array in the semiconductor memory device described in Embodiment 2can be used.

A row address signal is input to the first driver circuit 813_1. Thefirst driver circuit 813_1 selects a wiring arranged in a row directionin accordance with the row address signal input and sets the voltage ofthe wiring. The first driver circuit 813_1 includes a decoder, forexample. The decoder selects the wiring in accordance with the rowaddress signal input. Note that the semiconductor memory device in thisembodiment may include a plurality of first driver circuits 813_1.

A data signal and a column address signal are input to the second drivercircuit 813_2. The second driver circuit 813_2 sets the voltage of awiring arranged in a column direction. The second driver circuit 813_2includes a decoder, a plurality of analog switches, a read signal outputcircuit, and a read circuit, for example. The decoder selects thewiring. The plurality of analog switches determine whether or not thedata signal is output in accordance with a signal input from thedecoder. The read signal output circuit outputs a read signal to awiring serving as a data line or a selection line. The read circuitreads data stored in the memory cell 811 which is electrically connectedto a wiring selected by the read signal.

A write control signal, a read control signal, and an address signal areinput to the drive control circuit 813_3. The drive control circuit813_3 generates and outputs signals which control the operation of thefirst driver circuit 813_1 and the second driver circuit 813_2 inaccordance with the input write control signal, read control signal, andaddress signal. For example, the drive control circuit 813_3 outputs aplurality of row address signals to the first driver circuit 813_1 and aplurality of column address signals to the second driver circuit 813_2in accordance with the address signal.

As described with reference to FIG. 7, the example of the memory devicein this embodiment includes a memory cell array including a plurality ofmemory cells, a first driver circuit (also referred to as IDRV), asecond driver circuit (also referred to as JDRV), and a drive controlcircuit.

With such a structure, data can be written to and read from each memorycell.

Embodiment 8

In this embodiment, examples of electronic devices each including thesemiconductor memory device in the above embodiment are described.

Examples of structures of electronic devices in this embodiment aredescribed with reference to FIGS. 8A to 8D.

The electronic device illustrated in FIG. 8A is an example of a personaldigital assistant. The personal digital assistant illustrated in FIG. 8Aincludes a housing 1001 a and a display portion 1002 a provided in thehousing 1001 a.

Note that a side surface 1003 a of the housing 1001 a may be providedwith a connection terminal for connecting the personal digital assistantto an external device and/or a button for operating the personal digitalassistant illustrated in FIG. 8A.

The personal digital assistant illustrated in FIG. 8A includes a CPU, amemory circuit, an interface for transmitting and receiving a signalbetween the external device and each of the CPU and the memory circuit,and an antenna for transmitting and receiving a signal to and from theexternal device, in the housing 1001 a.

The personal digital assistant illustrated in FIG. 8A serves as one ormore of a telephone set, an e-book reader, a personal computer, and agame machine, for example.

The electronic device illustrated in FIG. 8B is an example of a foldingpersonal digital assistant. The personal digital assistant illustratedin FIG. 8B includes a housing 1001 b, a display portion 1002 b providedin the housing 1001 b, a housing 1004 b, a display portion 1005 bprovided in the housing 1004 b, and a hinge 1006 b for connecting thehousing 1001 b and the housing 1004 b.

In the personal digital assistant illustrated in FIG. 8B, the housing1001 b can be stacked on the housing 1004 b by moving the housing 1001 bor the housing 1004 b with the hinge 1006 b.

Note that a side surface 1003 b of the housing 1001 b or a side surface1007 b of the housing 1004 b may be provided with a connection terminalfor connecting the personal digital assistant to an external deviceand/or a button for operating the personal digital assistant illustratedin FIG. 8B.

The display portion 1002 b and the display portion 1005 b may displaydifferent images or one image. Note that the display portion 1005 b isnot necessarily provided, and a keyboard which is an input device may beprovided instead of the display portion 1005 b.

The personal digital assistant illustrated in FIG. 8B includes a CPU, amemory circuit, and an interface for transmitting and receiving a signalbetween the external device and each of the CPU and the memory circuitin the housing 1001 b or the housing 1004 b. Note that the personaldigital assistant illustrated in FIG. 8B may include an antenna fortransmitting and receiving a signal to and from the external device.

The personal digital assistant illustrated in FIG. 8B serves as one ormore of a telephone set, an e-book reader, a personal computer, and agame machine, for example.

The electronic device illustrated in FIG. 8C is an example of astationary information terminal. The stationary information terminalillustrated in FIG. 8C includes a housing 1001 c and a display portion1002 c provided in the housing 1001 c.

Note that the display portion 1002 c can be provided on a deck portion1008 c in the housing 1001 c.

The stationary information terminal illustrated in FIG. 8C includes aCPU, a memory circuit, and an interface for transmitting and receiving asignal between the external device and each of the CPU and the memorycircuit in the housing 1001 c. Note that the stationary informationterminal illustrated in FIG. 8C may include an antenna for transmittingand receiving a signal to and from the external device.

Further, a side surface 1003 c of the housing 1001 c in the stationaryinformation terminal illustrated in FIG. 8C may be provided with one ormore of a ticket output portion that outputs a ticket or the like, acoin slot, and a bill slot.

The stationary information terminal illustrated in FIG. 8C serves as anautomated teller machine, an information communication terminal (alsoreferred to as a multimedia station) for ordering a ticket or the like,or a game machine, for example.

The electronic device illustrated in FIG. 8D is an example of astationary information terminal. The stationary information terminalillustrated in FIG. 8D includes a housing 1001 d and a display portion1002 d provided in the housing 1001 d. Note that a support forsupporting the housing 1001 d may also be provided.

Note that a side surface 1003 d of the housing 1001 d may be providedwith a connection terminal for connecting the stationary informationterminal to an external device and/or a button for operating thestationary information terminal illustrated in FIG. 8D.

The stationary information terminal illustrated in FIG. 8D may include aCPU, a memory circuit, and an interface for transmitting and receiving asignal between the external device and each of the CPU and the memorycircuit in the housing 1001 d. Note that the stationary informationterminal illustrated in FIG. 8D may include an antenna for transmittingand receiving a signal to and from the external device.

The stationary information terminal illustrated in FIG. 8D serves as adigital photo frame, a monitor, or a television set, for example.

The semiconductor memory device in the above embodiment is used as onememory circuit in an electronic device, for example. For example, thesemiconductor memory device in the above embodiment is used as one ofthe memory circuits in the electronic devices illustrated in FIGS. 8A to8D.

As described with reference to FIGS. 8A to 8D, the examples of theelectronic devices in this embodiment each include a memory circuitwhich includes the semiconductor memory device in the above embodiment.

With such a structure, even when power is not supplied, data in anelectronic device can be retained for a certain period. Thus,reliability can be improved and power consumption can be reduced.

Further, without limitation to the structures illustrated in FIGS. 8A to8D, a portable semiconductor memory device or the like provided with aconnector can be manufactured using the semiconductor memory device inthe above embodiment.

A specific example is described below in which the semiconductor memorydevice in the above embodiment is applied to a portable device such as apersonal digital assistant, a mobile phone, a smartphone, or an e-bookreader.

In portable devices, SRAMs or DRAMs are used for temporary storage ofimage data, or the like.

The reason for using an SRAM or a DRAM is that a flash memory has lowresponse speed and is unsuitable for image processing.

However, in the case of using an SRAM or a DRAM for temporary storage ofimage data, there is a problem described below.

An SRAM has the advantage of a high response speed. In a usual SRAM, asillustrated in FIG. 9A, one memory cell includes six transistors 1001,1002, 1003, 1004, 1005, and 1006, which are driven by an X decoder 1007and a Y decoder 1008. Moreover, in the usual SRAM, high speed driving ismade possible by a first inverter including the transistor 1003 and thetransistor 1005 and a second inverter including the transistor 1004 andthe transistor 1006.

However, the usual SRAM has the disadvantage of a large cell areabecause one memory cell is formed with six transistors. Provided thatthe minimum feature size of a design rule is F, the memory cell area ofan SRAM is generally 100 F² to 150 F². Therefore, the SRAM has a problemin that a unit price per bit is the most expensive among memory devices.

On the other hand, in a DRAM, as illustrated in FIG. 9B, one memory cellincludes a transistor 1111 and a storage capacitor 1112, which aredriven by an X decoder 1113 and a Y decoder 1114. One memory cell can beformed with one transistor and one capacitor, whereby the cell area issmall. The memory cell area of the DRAM is usually 10 F² or less.However, the DRAM has a problem in that a refresh operation needs to befrequently performed and power is consumed even when different data isnot rewritten.

Unlike the SRAM and the DRAM, the semiconductor memory device in theabove embodiment has a memory cell area of approximately 10 F² and doesnot need frequent refresh operations.

With the use of the semiconductor memory device in the above embodiment,power consumption can be reduced while an increase in the area of amemory cell is suppressed; therefore, the above two problems can besolved.

FIG. 10 is a block diagram illustrating an example of a portable device.The portable device illustrated in FIG. 10 includes, for example, aradio frequency (RF) circuit 1201, an analog baseband circuit 1202, adigital baseband circuit 1203, a battery 1204, a power supply circuit1205, an application processor 1206, a memory 1210 that is a flashmemory, a display controller 1211, a memory 1212, a display 1213, atouch sensor 1219, an audio circuit 1217 (such as a speaker or amicrophone), a keyboard 1218 that is one of input means, and the like.

The RF circuit 1201 receives an electric wave including data, forexample. For example, an antenna or the like is used as the RF circuit1201.

By providing the touch sensor 1219, a display portion 1214 of thedisplay 1213 can be operated.

The display 1213 includes the display portion 1214, a source driver1215, and a gate driver 1216. Operation of the display portion 1214 iscontrolled by the source driver 1215 and the gate driver 1216.

The application processor 1206 includes a CPU 1207, a digital signalprocessor (also referred to as a DSP) 1208, and an interface (alsoreferred to as an IF) 1209.

The memory 1212 usually includes an SRAM or a DRAM; however, when thesemiconductor memory device in the above embodiment is used, a unitprice per bit of a memory can be reduced and power consumption of thememory 1212 can be reduced.

An example of a structure of the memory 1212 to which the semiconductormemory device in the above embodiment is applied is illustrated in FIG.11. A memory illustrated in FIG. 11 includes a memory device 1302 and amemory device 1303 to which the semiconductor memory device in the aboveembodiment is applied, a switch 1304, a switch 1305, and a memorycontroller 1301.

Further, an example of operation of the portable device illustrated inFIG. 10 in which the memory illustrated in FIG. 11 is employed as thememory 1212 is described.

First, an image is formed as a result of reception of an electric waveincluding data, or by the application processor 1206. The formed imageis stored in the memory device 1302 as data through the switch 1304.Then, the data stored in the memory device 1302 is output to the displaycontroller 1211 through the switch 1305 and then to the display 1213; animage based on the input image data is displayed by the display 1213. Inthe case where the image is not changed, the data is read from thememory device 1302 at a frequency of, usually, higher than or equal to60 Hz and lower than or equal to 130 Hz, and the read data iscontinuously sent to the display controller 1211 through the switch1305. In the case where the user carries out an operation to rewrite theimage, a new image is formed by the application processor 1206, and theimage is stored in the memory device 1303 as data through the switch1304. Also during this step, the image data is regularly read from thememory device 1302 through the switch 1305. After the new image data isstored in the memory device 1303, in a subsequent frame period of thedisplay 1213, the data stored in the memory device 1303 is read and theread data is output to the display 1213 through the switch 1305 and thedisplay controller 1211. The display 1213 to which the data is inputdisplays an image based on the input image data. The above-describedread operation is repeated until when next data is stored in the memorydevice 1302. Data is written to and read from the memory device 1302 andthe memory device 1303 alternately in this manner, whereby the display1213 displays an image.

Note that the memory device 1302 and the memory device 1303 are notlimited to different memory chips; one memory chip may be shared andused as the memory device 1302 and the memory device 1303.

As described above, when the semiconductor memory device in the aboveembodiment is used for the memory device 1302 and the memory device1303, a unit price of a memory can be reduced and power consumption canalso be reduced.

FIG. 12 is a block diagram illustrating an example of an e-book reader.In FIG. 12, for example, a battery 1401, a power supply circuit 1402, amicroprocessor 1403, a memory 1404 that is a flash memory, an audiocircuit 1405, a keyboard 1406, a memory 1407, a touch panel 1408, adisplay 1409, and a display controller 1410 are included.

The microprocessor 1403 includes, for example, a CPU 1403 a, a DSP 1403b, and an IF 1403 c.

For example, the semiconductor memory device in the above embodiment canbe used for the memory 1407. The memory 1407 temporarily retains thecontents of books as data.

A function of the memory 1407 can be applied to, for example, the casewhere the user utilizes a highlighting function. For example, when theuser reads an e-book, the user will put a mark on a specific part insome cases. Such a marking function is called a highlighting function,by which characters are changed in color or type, underlined, orbold-faced, for example, so that a specific part is made to lookdistinct from the other part. In the function, information about thepart specified by the user is stored and retained. In the case where theinformation is stored for a long time, the information may be copied tothe memory 1404.

When the semiconductor memory device in the above embodiment is used asthe memory 1407 as described above, a unit price of a memory can bereduced and power consumption can also be reduced.

EXAMPLE 1

A transistor in which an oxide semiconductor including In, Sn, and Zn asmain components is used as a channel formation region can have favorablecharacteristics by depositing the oxide semiconductor while heating asubstrate or by performing heat treatment after formation of an oxidesemiconductor film. Note that a main component refers to an elementincluded in a composition at 5 atomic % or higher.

By intentionally heating the substrate after formation of the oxidesemiconductor film including In, Sn, and Zn as main components, thefield-effect mobility of the transistor can be improved. Further, thethreshold voltage of the transistor can be positively shifted to makethe transistor normally off.

As an example, FIGS. 21A to 21C each show characteristics of atransistor in which an oxide semiconductor film including In, Sn, and Znas main components and having a channel length L of 3 μm and a channelwidth W of 10 μm, and a gate insulating layer with a thickness of 100 nmare used. Note that V_(d) was set to 10 V.

FIG. 21A shows characteristics of a transistor whose oxide semiconductorfilm including In, Sn, and Zn as main components was formed bysputtering without heating a substrate intentionally. The field-effectmobility of the transistor is 18.8 cm²/Vsec. On the other hand, when theoxide semiconductor film including In, Sn, and Zn as main components isformed while heating the substrate intentionally, the field-effectmobility can be improved. FIG. 21B shows characteristics of a transistorwhose oxide semiconductor film including In, Sn, and Zn as maincomponents was formed while heating a substrate at 200° C. Thefield-effect mobility of the transistor is 32.2 cm²/Vsec.

The field-effect mobility can be further improved by performing heattreatment after formation of the oxide semiconductor film including In,Sn, and Zn as main components. FIG. 21C shows characteristics of atransistor whose oxide semiconductor film including In, Sn, and Zn asmain components was formed by sputtering at 200° C. and then subjectedto heat treatment at 650° C. The field-effect mobility of the transistoris 34.5 cm²/Vsec.

The intentional heating of the substrate is expected to have anadvantageous effect of reducing moisture taken into the oxidesemiconductor film during the formation by sputtering. Further, the heattreatment after film formation enables hydrogen, a hydroxyl group, ormoisture to be released and removed from the oxide semiconductor film.In this manner, the field-effect mobility can be improved. Such animprovement in field-effect mobility is presumed to be achieved not onlyby removal of impurities by dehydration or dehydrogenation but also by areduction in interatomic distance due to an increase in density. Theoxide semiconductor can be crystallized by being purified by removal ofimpurities from the oxide semiconductor. In the case where such apurified non-single-crystal oxide semiconductor is used, ideally, afield-effect mobility exceeding 100 cm²/Vsec is expected to be realized.

The oxide semiconductor including In, Sn, and Zn as main components maybe crystallized in the following manner oxygen ions are implanted intothe oxide semiconductor; hydrogen, a hydroxyl group, or moistureincluded in the oxide semiconductor is released by heat treatment; andthe oxide semiconductor is crystallized through the heat treatment or byanother heat treatment performed later. By such crystallizationtreatment or recrystallization treatment, a non-single-crystal oxidesemiconductor having favorable crystallinity can be obtained.

The intentional heating of the substrate during film formation and/orthe heat treatment after the film formation contributes not only toimproving field-effect mobility but also to making the transistornormally off. In a transistor whose oxide semiconductor film includingIn, Sn, and Zn as main components, which is formed without heating asubstrate intentionally, is used as a channel formation region, thethreshold voltage tends to be shifted negatively. However, when theoxide semiconductor film formed while heating the substrateintentionally is used, the problem of the negative shift of thethreshold voltage can be solved. That is, the threshold voltage isshifted so that the transistor becomes normally off; this tendency canbe confirmed by comparison between FIGS. 21A and 21B.

Note that the threshold voltage can also be controlled by changing theratio of In, Sn, and Zn; when the composition ratio of In, Sn, and Zn is2:1:3, a normally-off transistor is expected to be formed. In addition,a highly crystalline oxide semiconductor film can be obtained by settingthe composition ratio of a target as follows:In:Sn:Zn=2:1:3.

The temperature of the intentional heating of the substrate or thetemperature of the heat treatment is 150° C. or higher, preferably 200°C. or higher, or more preferably 400° C. or higher. When film formationor heat treatment is performed at a high temperature, the transistor canbe normally off.

By intentionally heating the substrate during film formation and/or byperforming heat treatment after the film formation, the stabilityagainst a gate-bias stress can be increased. For example, when a gatebias is applied with an intensity of 2 MV/cm at 150° C. for 1 hour,drift of the threshold voltage can be less than ±1.5 V, preferably lessthan ±1.0 V.

A BT test was performed on the following two transistors: Sample 1 onwhich heat treatment was not performed after formation of an oxidesemiconductor film; and Sample 2 on which heat treatment at 650° C. wasperformed after formation of an oxide semiconductor film.

First, V_(g)-I_(d) characteristics of the transistors were measured at asubstrate temperature of 25° C. and V_(d) of 10 V. Then, the substratetemperature was set to 150° C. and V_(d) was set to 0.1 V. After that,V_(g) of 20 V was applied so that the intensity of an electric fieldapplied to the gate insulating layer was 2 MV/cm, and the condition waskept for 1 hour. Next, V_(g) was set to 0 V. Then, V_(g)-I_(d)characteristics of the transistors were measured at a substratetemperature of 25° C. and V_(d) of 10 V. This process is called apositive BT test.

In a similar manner, first, V_(g)-I_(d) characteristics of thetransistors were measured at a substrate temperature of 25° C. and V_(d)of 10 V. Then, the substrate temperature was set to 150° C. and V_(d)was set to 0.1 V. After that, V_(g) of −20 V was applied so that theintensity of an electric field applied to the gate insulating layer was−2 MV/cm, and the condition was kept for 1 hour. Next, V_(g) was set to0 V. Then, V_(g)-I_(d) characteristics of the transistors were measuredat a substrate temperature of 25° C. and V_(d) of 10 V. This process iscalled a negative BT test.

FIGS. 22A and 22B show a result of the positive BT test of Sample 1 anda result of the negative BT test of Sample 1, respectively. FIGS. 23Aand 23B show a result of the positive BT test of Sample 2 and a resultof the negative BT test of Sample 2, respectively.

The amounts of shift in the threshold voltage of Sample 1 due to thepositive BT test and that due to the negative BT test were 1.80 V and−0.42 V, respectively. The amounts of shift in the threshold voltage ofSample 2 due to the positive BT test and that due to the negative BTtest were 0.79 V and 0.76 V, respectively. It is found that, in each ofSample 1 and Sample 2, the amount of shift in the threshold voltagebetween before and after the BT tests is small and the reliability ishigh.

The heat treatment can be performed in an oxygen atmosphere;alternatively, the heat treatment may be performed first in anatmosphere of nitrogen or an inert gas or under reduced pressure, andthen in an atmosphere including oxygen. Oxygen is supplied to the oxidesemiconductor after dehydration or dehydrogenation, whereby anadvantageous effect of the heat treatment can be further increased. As amethod for supplying oxygen after dehydration or dehydrogenation, amethod in which oxygen ions are accelerated by an electric field andimplanted into the oxide semiconductor film may be employed.

A defect due to oxygen deficiency is easily caused in the oxidesemiconductor or at an interface between the oxide semiconductor and astacked film; however, when excess oxygen is included in the oxidesemiconductor by the heat treatment, oxygen deficiency caused constantlycan be compensated for with excess oxygen. The excess oxygen is oxygenexisting between lattices. When the concentration of excess oxygen isset to higher than or equal to 1×10¹⁶/cm³ and lower than or equal to2×10²⁰/cm³, excess oxygen can be included in the oxide semiconductorwithout causing crystal distortion or the like.

When heat treatment is performed so that at least part of the oxidesemiconductor includes crystal, a more stable oxide semiconductor filmcan be obtained. For example, when an oxide semiconductor film which isformed by sputtering using a target having a composition ratio ofIn:Sn:Zn=1:1:1 without heating a substrate intentionally is analyzed byX-ray diffraction (XRD), a halo pattern is observed. The formed oxidesemiconductor film can be crystallized by being subjected to heattreatment. The temperature of the heat treatment can be set asappropriate; when the heat treatment is performed at 650° C., forexample, a clear diffraction peak can be observed in an X-raydiffraction analysis.

An XRD analysis of an In—Sn—Zn—O film was conducted. The XRD analysiswas conducted using an X-ray diffractometer D8 ADVANCE manufactured byBruker AXS, and measurement was performed by an out-of-plane method.

Sample A and Sample B were prepared and the XRD analysis was performedthereon. A method for manufacturing Sample A and Sample B will bedescribed below.

An In—Sn—Zn—O film with a thickness of 100 nm was formed over a quartzsubstrate that had been subjected to dehydrogenation treatment.

The In—Sn—Zn—O film was formed with a sputtering apparatus with a powerof 100 W (DC) in an oxygen atmosphere. An In—Sn—Zn—O target having anatomic ratio of In:Sn:Zn=1:1:1 was used as a target. Note that thesubstrate heating temperature in film formation was set at 200° C. Asample manufactured in this manner was used as Sample A.

Next, a sample manufactured by a method similar to that of Sample A wassubjected to heat treatment at 650° C. As the heat treatment, heattreatment in a nitrogen atmosphere was first performed for 1 hour andheat treatment in an oxygen atmosphere was further performed for 1 hourwithout lowering the temperature. A sample manufactured in this mannerwas used as Sample B.

FIG. 24 shows XRD spectra of Sample A and Sample B. No peak derived fromcrystal was observed in Sample A, whereas peaks derived from crystalwere observed when 2θ was around 35 deg. and 37 deg. to 38 deg. inSample B.

As described above, by intentionally heating the substrate duringdeposition of an oxide semiconductor including In, Sn, and Zn as maincomponents and/or by performing heat treatment after the deposition,characteristics of a transistor can be improved.

These substrate heating and heat treatment have an effect of preventinghydrogen and a hydroxyl group, which are adverse impurities for an oxidesemiconductor, from being included in the film or an effect of removinghydrogen and a hydroxyl group from the film. That is, an oxidesemiconductor can be purified by removing hydrogen serving as a donorimpurity from the oxide semiconductor, whereby a normally-off transistorcan be obtained. The high purification of an oxide semiconductor enablesthe off-state current of the transistor to be 1 aA/μm or lower. Here,the unit of the off-state current represents current per micrometer of achannel width.

FIG. 25 shows a relation between the off-state current of a transistorand the inverse of substrate temperature (absolute temperature) atmeasurement. Here, for simplicity, the horizontal axis represents avalue (1000/T) obtained by multiplying an inverse of substratetemperature at measurement by 1000.

Specifically, as shown in FIG. 25, the off-state current can be 1 aA/μm(1×10⁻¹⁸ A/μm) or lower, 100 zA/μm (1×10⁻¹⁹ A/μm) or lower, and 1 zA/μm(1×10⁻²¹ A/μm) or lower when the substrate temperature is 125° C., 85°C., and room temperature (27° C.), respectively. Preferably, theoff-state current can be 0.1 aA/μm (1×10⁻¹⁹ A/μm) or lower, 10 zA/μm(1×10⁻²⁰ A/μm) or lower, and 0.1 zA/μm (1×10⁻²² A/μm) or lower at 125°C., 85° C., and room temperature, respectively. The above values ofoff-state currents are clearly much lower than that of the transistorusing Si as a semiconductor film.

Note that in order to prevent hydrogen and moisture from being includedin the oxide semiconductor film during formation thereof, it ispreferable to increase the purity of a sputtering gas by sufficientlysuppressing leakage from the outside of a film formation chamber anddegasification through an inner wall of the film formation chamber. Forexample, a gas with a dew point of −70° C. or lower is preferably usedas the sputtering gas in order to prevent moisture from being includedin the film. In addition, it is preferable to use a target which ispurified so as not to include impurities such as hydrogen and moisture.Although it is possible to remove moisture from a film of an oxidesemiconductor including In, Sn, and Zn as main components by heattreatment, a film which does not include moisture originally ispreferably formed because moisture is released from the oxidesemiconductor including In, Sn, and Zn as main components at a highertemperature than from an oxide semiconductor including In, Ga, and Zn asmain components.

The relation between the substrate temperature and electriccharacteristics of a transistor of a sample, on which heat treatment at650° C. was performed after formation of the oxide semiconductor film,was evaluated.

The transistor used for the measurement has a channel length L of 3 μm,a channel width W of 10 μm, Lov of 0 μm, and dW of 0 μm. Note that V_(d)was set to 10 V. Note that the substrate temperature was −40° C., −25°C., 25° C., 75° C., 125° C., and 150° C. Here, in a transistor, thewidth of a portion where a gate electrode overlaps with one of a pair ofelectrodes is referred to as Lov, and the width of a portion of the pairof electrodes, which does not overlap with an oxide semiconductor film,is referred to as dW.

FIG. 26 shows the V_(g) dependence of I_(d) (a solid line) andfield-effect mobility (a dotted line). FIG. 27A shows a relation betweenthe substrate temperature and the threshold voltage, and FIG. 27B showsa relation between the substrate temperature and the field-effectmobility.

From FIG. 27A, it is found that the threshold voltage gets lower as thesubstrate temperature increases. Note that the threshold voltage isdecreased from 1.09 V to −0.23 V in the range from −40° C. to 150° C.

From FIG. 27B, it is found that the field-effect mobility gets lower asthe substrate temperature increases. Note that the field-effect mobilityis decreased from 36 cm²/Vs to 32 cm²/Vs in the range from −40° C. to150° C. Thus, it is found that variation in electric characteristics issmall in the above temperature range.

In a transistor in which such an oxide semiconductor including In, Sn,and Zn as main components is used as a channel formation region, afield-effect mobility of 30 cm²/Vsec or higher, preferably 40 cm²/Vsecor higher, or more preferably 60 cm²/Vsec or higher can be obtained withthe off-state current maintained at 1 aA/μm or lower, which can achieveon-state current needed for an LSI. For example, in an FET where L/W is33 nm/40 nm, an on-state current of 12 μA or higher can flow when thegate voltage is 2.7 V and the drain voltage is 1.0 V. In addition,sufficient electric characteristics can be ensured in a temperaturerange needed for operation of a transistor. With such characteristics,an integrated circuit having a novel function can be realized withoutdecreasing the operating speed even when a transistor including an oxidesemiconductor is also provided in an integrated circuit formed using aSi semiconductor.

EXAMPLE 2

In this example, an example of a transistor in which an In—Sn—Zn—O filmis used as an oxide semiconductor film will be described with referenceto FIGS. 28A and 28B.

FIGS. 28A and 28B are a top view and a cross-sectional view of acoplanar transistor having a top-gate top-contact structure. FIG. 28A isthe top view of the transistor. FIG. 28B illustrates cross section A1-A2along dashed-dotted line A1-A2 in FIG. 28A.

The transistor illustrated in FIG. 28B includes a substrate 2000; a baseinsulating film 2002 provided over the substrate 2000; a protectiveinsulating film 2004 provided in the periphery of the base insulatingfilm 2002; an oxide semiconductor film 2006 provided over the baseinsulating film 2002 and the protective insulating film 2004 andincluding a high-resistance region 2006 a and low-resistance regions2006 b; a gate insulating layer 2008 provided over the oxidesemiconductor film 2006; a gate electrode 2010 provided so as to overlapwith the oxide semiconductor film 2006 with the gate insulating layer2008 interposed therebetween; a sidewall insulating film 2012 providedin contact with a side surface of the gate electrode 2010; a pair ofelectrodes 2014 provided in contact with at least the low-resistanceregions 2006 b; an interlayer insulating film 2016 provided so as tocover at least the oxide semiconductor film 2006, the gate electrode2010, and the pair of electrodes 2014; and a wiring 2018 provided so asto be connected to at least one of the pair of electrodes 2014 throughan opening formed in the interlayer insulating film 2016.

Although not illustrated, a protective film may be provided so as tocover the interlayer insulating film 2016 and the wiring 2018. With theprotective film, a minute amount of leakage current generated by surfaceconduction of the interlayer insulating film 2016 can be reduced andthus the off-state current of the transistor can be reduced.

EXAMPLE 3

In this example, another example of a transistor in which an In—Sn—Zn—Ofilm is used as an oxide semiconductor film will be described.

FIGS. 29A and 29B are a top view and a cross-sectional view whichillustrate a structure of a transistor manufactured in this example.FIG. 29A is the top view of the transistor. FIG. 29B is across-sectional view along dashed-dotted line B1-B2 in FIG. 29A.

The transistor illustrated in FIG. 29B includes a substrate 2100; a baseinsulating film 2102 provided over the substrate 2100; an oxidesemiconductor film 2106 provided over the base insulating film 2102; apair of electrodes 2114 provided in contact with the oxide semiconductorfilm 2106; a gate insulating layer 2108 provided over the oxidesemiconductor film 2106 and the pair of electrodes 2114; a gateelectrode 2110 provided so as to overlap with the oxide semiconductorfilm 2106 with the gate insulating layer 2108 interposed therebetween;an interlayer insulating film 2116 provided so as to cover the gateinsulating layer 2108 and the gate electrode 2110; wirings 2118connected to the pair of electrodes 2114 through openings formed in theinterlayer insulating film 2116; and a protective film 2120 provided soas to cover the interlayer insulating film 2116 and the wirings 2118.

As the substrate 2100, a glass substrate was used. As the baseinsulating film 2102, a silicon oxide film was used. As the oxidesemiconductor film 2106, an In—Sn—Zn—O film was used. As the pair ofelectrodes 2114, a tungsten film was used. As the gate insulating layer2108, a silicon oxide film was used. The gate electrode 2110 had astacked-layer structure of a tantalum nitride film and a tungsten film.The interlayer insulating film 2116 had a stacked-layer structure of asilicon oxynitride film and a polyimide film. The wirings 2118 each hada stacked-layer structure in which a titanium film, an aluminum film,and a titanium film were formed in this order. As the protective film2120, a polyimide film was used.

Note that in the transistor having the structure illustrated in FIG.29A, the width of a portion where the gate electrode 2110 overlaps withone of the pair of electrodes 2114 is referred to as Lov. Similarly, thewidth of a portion where the pair of electrodes 2114, which does notoverlap with the oxide semiconductor film 2106, is referred to as dW.

This application is based on Japanese Patent Application serial No.2010-292496 filed with Japan Patent Office on Dec. 28, 2010 and JapanesePatent Application serial No. 2011-112538 filed with Japan Patent Officeon May 19, 2011, the entire contents of which are hereby incorporated byreference.

What is claimed is:
 1. A semiconductor device comprising a memorycircuit, the memory circuit comprising: a first transistor; a secondtransistor; and a rectifier element comprising a pair of currentterminals with a first region having a first conductivity typesemiconductor and a second region having a second, differentconductivity type semiconductor interposed therebetween, wherein thefirst region and the second region are in contact with each other,wherein one of a source and a drain of the first transistor isconfigured to receive a data signal, wherein a gate of the secondtransistor is electrically connected to the other of the source and thedrain of the first transistor, wherein one of the current terminals iselectrically connected to a source or a drain of the second transistor,wherein the first transistor comprises an oxide semiconductor layer,wherein a first channel formation region is formed within the oxidesemiconductor layer, and wherein the first transistor is formed over thesecond transistor and the rectifier.
 2. The semiconductor deviceaccording to claim 1, wherein the second transistor comprises asemiconductor layer comprising silicon, and wherein a second channelformation region is formed within the semiconductor layer.
 3. Thesemiconductor device according to claim 1, wherein the oxidesemiconductor layer comprises a pair of regions which are separated fromeach other, to which a dopant is added, and between which the firstchannel formation region is formed.
 4. The semiconductor deviceaccording to claim 1, wherein the first channel formation regioncomprises In, Ga, and Zn, wherein the first channel formation regioncomprises a crystalline structure, wherein the first channel formationregion comprises a phase having a triangular, hexagonal, regulartriangular, or regular hexagonal atomic arrangement when seen from adirection perpendicular to an a-b plane, and wherein in the firstchannel formation region, metal atoms are arranged in a layered mannerwhen seen from a direction perpendicular to a c-axis direction or metalatoms and oxygen atoms are arranged in a layered manner when seen fromthe direction perpendicular to the c-axis direction.
 5. A semiconductormemory device comprising: a plurality of memory cells arranged in amatrix; first wirings; second wirings; and third wirings, each of theplurality of memory cells comprising: a first transistor; a secondtransistor; and a rectifier element comprising a pair of currentterminals with a first region having a first conductivity typesemiconductor and a second region having a second, differentconductivity type semiconductor interposed therebetween, wherein thefirst region and the second region are in contact with each other;wherein one of a source and a drain of the first transistor iselectrically connected to one of the first wirings, wherein a gate ofthe first transistor is electrically connected to one of the secondwirings, wherein one of a source and a drain of the second transistor iselectrically connected to one of the third wirings, wherein a gate ofthe second transistor is electrically connected to the other of thesource and the drain of the first transistor, wherein one of the currentterminals is electrically connected to the other of the source and thedrain of the second transistor, wherein the other of the currentterminals is electrically connected to a corresponding one of the firstwirings, wherein the first transistor comprises an oxide semiconductorlayer, wherein a first channel formation region is formed within theoxide semiconductor layer, and wherein the first transistor is formedover the second transistor and the rectifier.
 6. The semiconductormemory device according to claim 5, wherein the second transistorcomprises a semiconductor layer comprising silicon, and wherein a secondchannel formation region is formed within the semiconductor layer. 7.The semiconductor memory device according to claim 5, wherein the oxidesemiconductor layer comprises a pair of regions which are separated fromeach other, to which a dopant is added, and between which the firstchannel formation region is formed.
 8. The semiconductor memory deviceaccording to claim 5, wherein the first channel formation regioncomprises In, Ga, and Zn, wherein the first channel formation regioncomprises a crystalline portion, wherein the first channel formationregion comprises a phase having a triangular, hexagonal, regulartriangular, or regular hexagonal atomic arrangement when seen from adirection perpendicular to an a-b plane, and wherein in the firstchannel formation region, metal atoms are arranged in a layered mannerwhen seen from a direction perpendicular to a c-axis direction or metalatoms and oxygen atoms are arranged in a layered manner when seen fromthe direction perpendicular to the c-axis direction.